Internal Medicine

3151 questions

All types3,151 Multiple choice1,611 Multiple select73 Numeric81 Matching714 Ordering672

A 22-year-old woman is brought to the emergency department by her roommate due to agitation, nausea, and ringing in her ears. The roommate reports finding several empty over-the-counter medication bottles in their bathroom. On physical examination, the patient appears distressed and is breathing rapidly.

Temperature is $38.1^\circ$ C ( $100.6^\circ$ F), blood pressure is 112/68 mm Hg, pulse is 118/min, and respiratory rate is 26/min. Laboratory studies show:

Component	Value
Serum $Na^+$	142 mEq/L
Serum $K^+$	4.1 mEq/L
Serum $Cl^-$	102 mEq/L
Serum $HCO_3^-$	12 mEq/L
BUN	18 mg/dL
Serum creatinine	0.9 mg/dL

Arterial blood gas on room air:

Component	Value
pH	7.41
$PaCO_2$	18 mm Hg
$PaO_2$	95 mm Hg

Which of the following best characterizes this patient's underlying acid-base disturbance?

Concomitant primary high anion gap metabolic acidosis and primary respiratory alkalosis

Primary high anion gap metabolic acidosis with appropriate respiratory compensation

Primary respiratory alkalosis with appropriate metabolic compensation

Concomitant primary non-anion gap metabolic acidosis and primary respiratory alkalosis

Concomitant primary high anion gap metabolic acidosis and primary metabolic alkalosis

Show answer & explanation

Answer: Concomitant primary high anion gap metabolic acidosis and primary respiratory alkalosis

Answer

Concomitant primary high anion gap metabolic acidosis and primary respiratory alkalosis

The correct answer identifies a classic mixed acid-base disorder typically seen in salicylate (aspirin) toxicity. Initially, salicylates directly stimulate the medullary respiratory center, causing hyperventilation and a primary respiratory alkalosis. Shortly after, they uncouple oxidative phosphorylation, leading to anaerobic metabolism, accumulation of organic acids (lactic acid, ketoacids), and a primary high anion gap metabolic acidosis. The concurrent primary acidosis and primary alkalosis pull the pH in opposite directions, often resulting in a near-normal or slightly alkalemic pH (e.g., 7.41), while both the

HCO_3^-

and

PaCO_2

are markedly reduced beyond simple compensatory mechanisms.

Step-by-Step Solution

Calculate the serum anion gap.

Anion Gap =

Na^+ - (Cl^- + HCO_3^-)

142 - (102 + 12)

= 28 mEq/L.

Establishing the anion gap is the critical first step in evaluating any acid-base disturbance. An AG > 12 mEq/L indicates a primary High Anion Gap Metabolic Acidosis (HAGMA).

Calculate the expected respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2

. Range is 24 to 28 mm Hg.

Winter's formula determines if the respiratory response (change in

PaCO_2

) is purely compensatory for the metabolic acidosis, or if an independent respiratory disorder exists.

Compare the actual

PaCO_2

to the expected

PaCO_2

Actual

PaCO_2

(18 mm Hg) < Expected

PaCO_2

(24-28 mm Hg).

Because the measured

PaCO_2

is lower than the expected compensatory range, there must be a concurrent primary process actively blowing off

CO_2

. This establishes the diagnosis of a primary respiratory alkalosis.

Calculate the Delta-Delta ratio to check for hidden metabolic disorders.

\Delta AG = 28 - 12 = 16

\Delta HCO_3^- = 24 - 12 = 12

. Ratio =

16/12 = 1.33

A ratio between 1 and 2 confirms an uncomplicated HAGMA with no additional hidden metabolic alkalosis or non-anion gap metabolic acidosis (NAGMA).

Key Concept

Mixed acid-base disorders in salicylate toxicity

Question 1662Question →

A 68-year-old man presents to the clinic with a 2-year history of progressive right knee pain. The pain typically worsens at the end of the day or after prolonged walking, and it improves with rest. He reports about 10 to 15 minutes of joint stiffness in the morning. His medical history is significant for hypertension. Physical examination reveals bony enlargement, decreased range of motion, and crepitus of the right knee without significant effusion, erythema, or warmth. Radiographs show medial joint space narrowing, subchondral sclerosis, and osteophyte formation. Arrange the following interventions in the generally recommended, step-wise sequence for the management of this patient's condition, assuming an inadequate response to each preceding step.

Drag items to arrange them in the correct order

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Answer

The correct sequence begins with non-pharmacologic measures (physical therapy/weight loss), followed by topical NSAIDs, oral NSAIDs, intra-articular glucocorticoid injections, and ultimately total knee arthroplasty.

The standard escalation of therapy for knee osteoarthritis begins with the least invasive and lowest-risk interventions. Non-pharmacologic therapies (physical therapy, weight loss) are universally the first step. If pharmacologic therapy is needed, topical NSAIDs are preferred for initial management in superficial joints like the knee, especially in patients over 65 or those with comorbidities (like hypertension), due to a much lower risk of systemic side effects compared to oral NSAIDs. When topical agents fail, oral NSAIDs are the next step. If symptoms persist despite oral therapy, intra-articular glucocorticoid injections offer targeted relief. Finally, when all conservative and medical modalities have been exhausted and the patient's quality of life is severely impacted, referral for total joint arthroplasty is the definitive treatment.

Step-by-Step Solution

Diagnose the condition based on the clinical presentation and radiographic findings.

The patient has classical features of knee osteoarthritis (progressive mechanical pain, short morning stiffness, crepitus, osteophytes).

Establishing the correct diagnosis determines the management pathway.

Determine the first step in management.

Physical therapy and weight loss counseling should be initiated.

Non-pharmacologic therapy is the universal first-line intervention for osteoarthritis.

Determine the safest initial pharmacologic intervention.

Topical diclofenac is prescribed.

Topical NSAIDs provide localized pain relief with minimal systemic absorption, making them ideal first-line drugs for superficial joints, particularly in older patients with comorbidities like hypertension.

Determine the next step if topical therapy fails.

Oral naproxen is prescribed.

Oral NSAIDs are more potent but carry systemic risks; they are the next line of therapy.

Determine the intervention for refractory pain prior to surgery.

Intra-articular glucocorticoid injection is administered.

Injections provide direct, potent localized anti-inflammatory effects for patients failing systemic therapy or requiring bridge therapy before surgery.

Determine the definitive treatment if all else fails.

Referral for total knee arthroplasty.

Joint replacement is indicated for severe, end-stage osteoarthritis severely impacting quality of life.

Key Concept

The step-wise management of knee osteoarthritis prioritizes low-risk, non-pharmacologic interventions first, escalating progressively to topical agents, oral agents, intra-articular injections, and finally surgery.

Question 1663Question →

A 38-year-old man presents to the clinic complaining of progressive fatigue, generalized weakness, and cold intolerance over the past 8 months. He also reports a significantly decreased libido. Two years ago, he was involved in a severe motor vehicle collision that resulted in a basilar skull fracture and a prolonged intensive care unit stay. He takes no daily medications. His temperature is $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ), pulse is 55/min, and blood pressure is 102/64 mmHg. Physical examination demonstrates pale, dry skin and a delayed relaxation phase of the ankle reflexes. Visual field testing is normal.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.2 \text{ \mu U/mL}$	$0.5-5.0 \text{ \mu U/mL}$
Free thyroxine (T4)	$0.4 \text{ ng/dL}$	$0.9-1.7 \text{ ng/dL}$
Total testosterone	$140 \text{ ng/dL}$	$300-1000 \text{ ng/dL}$
Luteinizing hormone (LH)	$1.1 \text{ mIU/mL}$	$2.0-12.0 \text{ mIU/mL}$

Which of the following is the most likely underlying mechanism for this patient's condition?

Autoimmune destruction of thyroid follicular cells

Primary failure of testicular Leydig cells

Disruption of the hypothalamic-pituitary axis

Peripheral tissue resistance to thyroid hormone

Autonomous production of thyroid hormone

Show answer & explanation

Answer: Disruption of the hypothalamic-pituitary axis

Answer

Disruption of the hypothalamic-pituitary axis

The correct answer identifies that the patient's combined laboratory findings and history point to a central endocrine defect. The patient has a low Free T4, but his TSH is not elevated (it is inappropriately normal). This indicates a failure of the pituitary to respond to low circulating thyroid hormone levels, characteristic of secondary hypothyroidism. Similarly, his testosterone is low without an appropriate compensatory rise in LH, indicating secondary hypogonadism. Given his history of severe traumatic brain injury and basilar skull fracture, the most likely mechanism is disruption of the hypothalamic-pituitary axis (traumatic hypopituitarism), leading to multiple anterior pituitary hormone deficiencies.

Step-by-Step Solution

Analyze the patient's presenting symptoms and physical exam findings.

The patient exhibits classic signs of hypothyroidism (fatigue, cold intolerance, delayed reflexes) and hypogonadism (decreased libido).

Identifying the clinical syndromes guides the interpretation of the subsequent laboratory data.

Interpret the thyroid function tests.

Free T4 is low, but TSH is inappropriately normal rather than elevated, indicating central (secondary) hypothyroidism.

To distinguish between primary thyroid failure and a central defect.

Interpret the gonadal function tests.

Testosterone is low, but LH is inappropriately normal rather than elevated, indicating central (secondary) hypogonadism.

To determine if the hypogonadism is due to primary testicular failure or a central defect.

Synthesize the laboratory findings with the patient's medical history.

The presence of multiple central endocrine deficiencies in a patient with a history of severe traumatic brain injury points to post-traumatic hypopituitarism.

To determine the most likely underlying pathophysiologic mechanism.

Key Concept

Secondary (central) hypothyroidism and hypogonadism caused by traumatic brain injury (post-traumatic hypopituitarism).

Question 1664Question →

Emergency medical services arrive with a 74-year-old woman who became acutely lethargic at her assisted living facility. According to the staff, she had been complaining of severe, diffuse abdominal pain that was out of proportion to her physical exam findings earlier in the morning. Her medical history includes atrial fibrillation, type 2 diabetes mellitus, and severe peripheral vascular disease.

On evaluation, her blood pressure is 78/46 mm Hg, pulse is 132/min and irregularly irregular, and respirations are deep and rapid at 28/min. Physical examination reveals mottled, cool extremities and a rigid, diffusely tender abdomen.

Point-of-care laboratory testing reveals:
Serum $Na^+$ : 141 mEq/L
Serum $K^+$ : 4.9 mEq/L
Serum $Cl^-$ : 105 mEq/L
Serum $HCO_3^-$ : 14 mEq/L
Blood urea nitrogen: 32 mg/dL
Creatinine: 1.6 mg/dL
Lactate: 9.4 mmol/L (normal: 0.5-1.0 mmol/L)

Arterial blood gas on room air:
pH: 7.23
$PaO_2$ : 88 mm Hg

What is the expected arterial partial pressure of carbon dioxide ( $PaCO_2$ ) in mm Hg that indicates appropriate and isolated respiratory compensation for her primary acid-base disturbance?

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Answer: 29

Answer

The expected target

PaCO_2

is 29 mm Hg.

The patient presents with severe abdominal pain out of proportion to exam and an irregularly irregular pulse (atrial fibrillation), highly suggestive of acute mesenteric ischemia. This has led to profound lactic acidosis, confirmed by her low pH (7.23), low bicarbonate (14 mEq/L), and elevated lactate (9.4 mmol/L). This represents a primary high anion gap metabolic acidosis. To determine if this is an isolated disorder, the expected physiological respiratory compensation must be calculated using Winter's formula: Expected

PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2

. For this patient, the calculation is

(1.5 \times 14) + 8 = 21 + 8 = 29

mm Hg. If her measured

PaCO_2

matches this target (or falls within the 27 to 31 mm Hg range), it indicates an appropriate and isolated respiratory compensation without a superimposed primary respiratory disorder.

Step-by-Step Solution

Identify the primary acid-base disorder from the provided laboratory values.

The patient has a low pH (7.23) and a low serum bicarbonate (14 mEq/L), which establishes the presence of a primary metabolic acidosis.

Identifying the primary disturbance is required to select the correct physiological compensation formula.

Calculate the expected respiratory compensation.

Using Winter's formula: Expected

PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2

. Plugging in the values: Expected

PaCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29

mm Hg.

Winter's formula determines if the patient's respiratory response (hyperventilation) is adequate for the degree of metabolic acidosis, helping to rule out mixed acid-base disorders.

Key Concept

In the setting of a primary metabolic acidosis, the appropriate respiratory compensation is calculated using Winter's formula: Expected

PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2

Estimated Time:1m 0s

Question 1665Question →

Match each patient's clinical presentation and laboratory profile with the most characteristic finding expected on their microscopic urinalysis or urine sediment examination.

Click a left item, then click its matching right item

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Answer

The correct pairings match proton-pump inhibitor use with WBC casts (AIN); suspected multiple myeloma with glassy, fractured casts (cast nephropathy); ethylene glycol ingestion with envelope-shaped oxalate crystals; and tumor lysis syndrome with rhomboid uric acid crystals.

Each clinical scenario describes a classic etiology of acute kidney injury paired with a pathognomonic urine sediment finding. AIN presents with WBC casts, multiple myeloma cast nephropathy with fractured glassy casts, ethylene glycol toxicity with calcium oxalate crystals, and tumor lysis syndrome with uric acid crystals.

Step-by-Step Solution

Identify the underlying etiology of acute kidney injury for the patient on pantoprazole.

Diagnosed as drug-induced acute interstitial nephritis (AIN).

AIN is an allergic inflammatory response in the interstitium often triggered by PPIs or NSAIDs, presenting with WBC casts and eosinophils.

Identify the etiology for the elderly patient with a protein gap and anemia.

Diagnosed as multiple myeloma causing cast nephropathy.

Excess free light chains (Bence-Jones proteins) precipitate with Tamm-Horsfall mucoprotein, forming distinct dense, eosinophilic, glassy casts with fractured edges.

Analyze the acid-base derangement in the confused patient with an elevated osmolar gap.

Diagnosed as ethylene glycol toxicity.

Ethylene glycol breaks down into oxalate, which binds calcium in the urine to form pathognomonic envelope-shaped calcium oxalate crystals.

Determine the renal complication of Burkitt lymphoma treatment.

Diagnosed as acute uric acid nephropathy from tumor lysis syndrome.

Rapid tumor breakdown releases massive amounts of purines, converting to uric acid and forming rhomboid or rosette-shaped uric acid crystals.

Key Concept

Correlation of specific acute kidney injury (AKI) etiologies with their pathognomonic urine sediment findings.

Question 1666Question →

A 29-year-old man is evaluated in the outpatient office due to a one-year history of worsening sluggishness, weight gain, and diminished libido. He reports feeling constantly cold even when others are comfortable. His medical history is notable for a severe closed head injury sustained in a motorcycle collision three years ago, which required a two-week stay in the intensive care unit. He currently takes no medications. Vital signs show a temperature of $36.1^\circ\text{C}$ ( $97.0^\circ\text{F}$ ), blood pressure of $105/65 \text{ mm Hg}$ , and heart rate of $56/\text{min}$ . Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes. Testes are soft and measure $12 \text{ mL}$ bilaterally (normal $15-25 \text{ mL}$ ). Laboratory studies show:

Thyroid-stimulating hormone (TSH): $1.1 \text{ mU/L}$ (Reference: $0.5-5.0 \text{ mU/L}$ )
Free thyroxine (T4): $0.5 \text{ ng/dL}$ (Reference: $0.9-2.4 \text{ ng/dL}$ )
Total testosterone: $160 \text{ ng/dL}$ (Reference: $300-1000 \text{ ng/dL}$ )

Which of the following is the most likely diagnosis?

Autoimmune thyroiditis

Thyroxine-binding globulin deficiency

Anterior pituitary insufficiency

Non-thyroidal illness syndrome

Autoimmune polyglandular syndrome type 2

Show answer & explanation

Answer: Anterior pituitary insufficiency

Answer

Anterior pituitary insufficiency

This patient's presentation of fatigue, cold intolerance, delayed deep tendon reflexes, and low free T4 confirms a hypothyroid state. However, his TSH is strictly within normal limits. An intact hypothalamic-pituitary-thyroid axis would respond to a free T4 of

0.5 \text{ ng/dL}

with a robust elevation in TSH. The presence of a low free T4 paired with an inappropriately normal TSH indicates central (secondary) hypothyroidism. Concurrently, he has central hypogonadism (low libido, soft testes, low testosterone). Combined with his history of a severe traumatic brain injury (TBI), the most likely diagnosis is anterior pituitary insufficiency (post-traumatic hypopituitarism). TBI can cause shearing of the infundibular stalk or ischemic damage to the anterior pituitary gland, which may manifest months to years after the initial insult.

Step-by-Step Solution

Evaluate the clinical symptoms and physical exam findings.

The patient demonstrates classic signs of hypothyroidism (fatigue, cold intolerance, weight gain, bradycardia, delayed reflexes) and hypogonadism (diminished libido, small soft testes).

Identifying the affected endocrine axes guides the interpretation of the laboratory testing.

Interpret the laboratory hormone panel.

Free T4 is overtly low, but TSH is normal (

1.1 \text{ mU/L}

). Testosterone is low.

A low target hormone level with an inappropriately normal or low stimulating hormone level indicates secondary (central) failure rather than primary glandular failure.

Synthesize the laboratory pattern with the patient's medical history.

The patient has central hypothyroidism and central hypogonadism, which clinically correlates with a history of severe traumatic brain injury.

Head trauma can cause shearing forces on the hypothalamic-pituitary stalk or direct vascular injury, leading to post-traumatic hypopituitarism.

Key Concept

Secondary (central) hypothyroidism is characterized by a low free T4 in the setting of a low or inappropriately normal TSH. It can occur as a complication of severe traumatic brain injury.

Question 1667Question →

A 42-year-old man is brought to the emergency department by his husband due to a 2-day history of progressive confusion, fatigue, and fever. The patient had a mild upper respiratory tract infection one week ago but otherwise has no significant past medical history. He takes no medications.

Vital signs are:
Temperature: 38.6°C (101.5°F)
Blood pressure: 142/88 mm Hg
Heart rate: 110/min
Respiratory rate: 18/min
Oxygen saturation: 98% on room air

Physical examination reveals a lethargic man who is oriented to person only. Scleral icterus is present. The skin examination shows widespread pinpoint, non-blanching red macules over the lower extremities and abdomen. There is no nuchal rigidity, and the remainder of the neurologic examination is nonfocal.

Laboratory studies reveal:
Hemoglobin: 7.6 g/dL
Mean corpuscular volume (MCV): 88 fL
Platelets: 18,000/mm³
Leukocytes: 9,200/mm³
Blood urea nitrogen (BUN): 42 mg/dL
Creatinine: 2.3 mg/dL
Total bilirubin: 4.8 mg/dL
Indirect bilirubin: 3.9 mg/dL
Lactate dehydrogenase (LDH): 1,450 U/L
Haptoglobin: 8 mg/dL (Normal: 50–200 mg/dL)
Prothrombin time (PT): 12 seconds
Activated partial thromboplastin time (aPTT): 28 seconds
Direct antiglobulin (Coombs) test: Negative

Peripheral blood smear reveals numerous fragmented red blood cells.

Which of the following is the most appropriate next step in the management of this patient?

Transport the patient to radiology for a non-contrast computed tomography scan of the head

Transfuse one unit of apheresis platelets and closely monitor neurologic status

Initiate plasma exchange therapy

Administer high-dose intravenous corticosteroids and intravenous immunoglobulin

Obtain blood cultures and initiate empiric vancomycin and cefepime

Show answer & explanation

Answer: Initiate plasma exchange therapy

Answer

The most appropriate next step is to initiate plasma exchange therapy.

The patient's clinical presentation of fever, confusion, and petechiae, combined with laboratory evidence of thrombocytopenia, microangiopathic hemolytic anemia, and acute kidney injury, is highly characteristic of thrombotic thrombocytopenic purpura (TTP). TTP is caused by a severe deficiency of ADAMTS13, a metalloprotease that cleaves large von Willebrand factor (vWF) multimers. The accumulation of these large multimers leads to widespread platelet adhesion and aggregation, forming microthrombi that shear red blood cells and cause end-organ ischemia. Normal coagulation studies distinguish TTP from disseminated intravascular coagulation. The cornerstone of treatment is immediate plasma exchange, which removes the offending autoantibodies against ADAMTS13 and replenishes the missing enzyme.

Step-by-Step Solution

Analyze the patient's clinical presentation and vital signs.

The patient has fever, altered mental status, and petechiae, indicating a systemic process with neurologic and hematologic involvement.

To establish the primary clinical problem and assess for potential instability.

Evaluate the laboratory findings and peripheral blood smear.

Labs reveal profound thrombocytopenia and a microangiopathic hemolytic anemia (low Hb, high LDH, high indirect bilirubin, low haptoglobin, negative Coombs, and schistocytes). Coagulation studies (PT and aPTT) are normal.

To differentiate between causes of thrombocytopenia and hemolysis, such as TTP, HUS, DIC, or ITP.

Synthesize the findings to establish a diagnosis.

The combination of the classic pentad (microangiopathic hemolytic anemia, thrombocytopenia, fever, acute kidney injury, and neurologic changes) strongly supports a diagnosis of Thrombotic Thrombocytopenic Purpura (TTP).

TTP is a medical emergency requiring rapid clinical diagnosis based on the dyad of microangiopathic hemolytic anemia and thrombocytopenia without another apparent cause.

Determine the most appropriate immediate management based on the diagnosis.

The immediate, life-saving intervention for TTP is plasma exchange therapy (plasmapheresis).

Plasma exchange removes the ADAMTS13 autoantibodies and replaces the deficient enzyme, halting the formation of microthrombi.

Key Concept

The diagnosis and emergent management of Thrombotic Thrombocytopenic Purpura (TTP) using plasma exchange therapy.

Estimated Time:1m 30s

Question 1668Question →

A 38-year-old woman is admitted to the medical ward due to a sudden onset of severe headache, palpitations, and profuse diaphoresis. Her blood pressure is 185/115 mm Hg, and her heart rate is 118/min. She reports experiencing three similar episodes over the past month. The clinical presentation is highly suspicious for a catecholamine-secreting neuroendocrine tumor. Arrange the following steps in the most appropriate chronological sequence for the comprehensive diagnostic evaluation and perioperative management of this patient's suspected condition. Assume each step yields confirmatory findings that warrant progression to the subsequent step.

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Answer

The correct sequence requires biochemical confirmation first, followed by imaging localization, subsequent alpha-adrenergic blockade, subsequent beta-adrenergic blockade, and finally surgical resection.

The evaluation and management of pheochromocytoma mandate a strict, chronological protocol to ensure patient safety. The process begins with biochemical confirmation via plasma or urine metanephrines. Only after biochemical proof is established should cross-sectional imaging (CT/MRI) be obtained to localize the tumor. Perioperative medical management must follow a specific sequence: alpha-adrenergic blockade (e.g., phenoxybenzamine) is initiated 10-14 days prior to surgery to normalize blood pressure and expand intravascular volume. Beta-adrenergic blockade (e.g., propranolol) is subsequently added to control tachycardia, but only after adequate alpha-blockade is confirmed; premature beta-blockade causes unopposed alpha-stimulation and severe hypertensive crisis. Finally, after proper medical optimization, the patient undergoes surgical resection.

Step-by-Step Solution

Biochemical Screening

Measure plasma free metanephrines or 24-hour urine fractionated metanephrines.

Documenting catecholamine excess is required before attempting to localize a tumor due to the high background rate of benign, nonfunctioning adrenal incidentalomas.

Anatomical Localization

Perform a dedicated CT or MRI scan of the abdomen and pelvis.

Once biochemical evidence of a pheochromocytoma is established, imaging is required to locate the tumor and plan the surgical approach.

Initial Preoperative Pharmacotherapy

Initiate an alpha-adrenergic antagonist (e.g., phenoxybenzamine).

Alpha-blockade must be started 10-14 days before surgery to stabilize blood pressure fluctuations and restore the severely contracted intravascular volume characteristic of pheochromocytoma.

Secondary Preoperative Pharmacotherapy

Initiate a beta-adrenergic antagonist (e.g., propranolol).

Beta-blockade controls catecholamine-induced tachycardia but must strictly be administered only after adequate alpha-blockade has been achieved. Starting it earlier risks a catastrophic hypertensive crisis.

Definitive Treatment

Proceed with laparoscopic adrenalectomy.

Once the patient is hemodynamically optimized and medically prepared with dual blockade, surgical resection of the tumor can be safely performed.

Key Concept

The diagnostic and perioperative management sequence for pheochromocytoma requires strictly ordered progression: biochemical confirmation, anatomical localization, alpha-blockade, beta-blockade, and surgery.

Question 1669Question →

A 45-year-old male is evaluated in the emergency department for altered mental status. His spouse found him acting confused and stumbling in their basement next to several open containers of antifreeze. Vital signs are temperature $36.7^\circ C$ ( $98.1^\circ F$ ), blood pressure $115/78$ mm Hg, pulse $95$ /min, and respirations $26$ /min. He appears intoxicated but has no scent of alcohol on his breath. Laboratory results are as follows:

Test	Result
Sodium	$138$ mEq/L
Potassium	$4.5$ mEq/L
Chloride	$98$ mEq/L
Bicarbonate	$12$ mEq/L
Blood urea nitrogen	$14$ mg/dL
Creatinine	$1.5$ mg/dL
Glucose	$108$ mg/dL
Measured serum osmolality	$318$ mOsm/kg $H_2O$
Blood ethanol level	Undetectable

Based on these findings, what is the patient's serum osmolar gap?

Show answer & explanation

Answer: 31

Answer

The patient has a toxic alcohol ingestion (likely ethylene glycol from antifreeze), presenting with a high anion gap metabolic acidosis and an elevated osmolar gap. The osmolar gap is calculated as the difference between the measured serum osmolality and the calculated serum osmolality. With a calculated osmolality of 287 mOsm/kg H2O and a measured osmolality of 318 mOsm/kg H2O, the resulting gap is 31.

Step-by-Step Solution

Identify the values needed for the calculated osmolality formula.

Na = 138, BUN = 14, Glucose = 108.

These are the primary osmotically active solutes in the serum.

Calculate the expected serum osmolality.

Calculated Osmolality = (2 * 138) + (108 / 18) + (14 / 2.8) = 276 + 6 + 5 = 287 mOsm/kg H2O.

The standard formula accounts for sodium and its accompanying anions, plus urea and glucose.

Calculate the osmolar gap.

Osmolar Gap = 318 - 287 = 31.

The gap is the measured osmolality minus the calculated osmolality.

Key Concept

Calculation of the serum osmolar gap to identify the presence of unmeasured osmotically active substances, such as toxic alcohols.

Question 1670Question →

A male college student, aged $19$ , is brought to the emergency department after his roommate found him confused and vomiting. He has been complaining of abdominal pain for the past day. On physical examination, he appears lethargic and has dry mucous membranes. Vital signs demonstrate a heart rate of $120$ beats per minute and a blood pressure of $90/60$ mmHg. Point-of-care capillary glucose testing shows a level of $480$ mg/dL. An arterial blood gas analysis reveals a pH of $7.12$ . A dipstick urinalysis is strongly positive for ketones. The patient is diagnosed with diabetic ketoacidosis. Arrange the following interventions in the most appropriate chronological order for his initial management.

Drag items to arrange them in the correct order

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Answer

The correct sequence is: Administer normal saline, assess potassium level, initiate intravenous insulin, and finally transition to subcutaneous insulin.

In the management of diabetic ketoacidosis, the first and most critical step is volume expansion with isotonic saline to restore perfusion. Once fluids are running, the serum potassium must be checked. If potassium is severely low (

< 3.3

mEq/L), insulin must be delayed until potassium is repleted, because insulin shifts potassium intracellularly and can cause fatal arrhythmias. If potassium is safe, an intravenous regular insulin infusion is started to stop ketogenesis and close the anion gap. Once the ketoacidosis has resolved (closed anion gap) and the patient can eat, they are transitioned to subcutaneous maintenance insulin.

Step-by-Step Solution

Recognize the diagnosis of severe diabetic ketoacidosis with profound hypovolemia.

The patient requires immediate intravascular volume resuscitation.

Severe hyperglycemia causes osmotic diuresis, leading to profound dehydration and potential hypovolemic shock.

Prioritize interventions based on immediate life-threatening risks.

Volume resuscitation and electrolyte assessment take precedence over insulin administration.

Insulin given before adequate fluids can cause cardiovascular collapse. Insulin given before checking potassium can cause fatal arrhythmias.

Determine the appropriate sequence for insulin administration.

Intravenous insulin is used acutely, followed by subcutaneous insulin once the crisis has resolved.

Intravenous insulin allows for rapid titration to close the anion gap. Subcutaneous insulin is for maintenance once the patient is stable.

Key Concept

The initial management of diabetic ketoacidosis (DKA) prioritizes aggressive fluid resuscitation and potassium assessment before insulin administration.

Question 1671Question →

While rounding in the surgical intensive care unit, a physician is urgently called to the bedside of a 68-year-old male patient who abruptly developed respiratory distress. The patient underwent a right hemicolectomy for stage III colon cancer two days ago. Two hours prior to the acute event, his hemoglobin level was noted to be $6.8$ g/dL, prompting the initiation of a cross-matched packed red blood cell transfusion. Approximately $30$ minutes after the transfusion began, the patient reported the sudden onset of shaking chills and severe pain in his right flank.

Vital signs reveal a temperature of $39.2^\circ\text{C}$ ( $102.6^\circ\text{F}$ ), heart rate of $122/\text{min}$ , blood pressure of $85/50$ mm Hg, and respiratory rate of $24/\text{min}$ . Pulse oximetry shows an oxygen saturation of $94\%$ on room air.

Physical examination shows profound diaphoresis and anxiety. Jugular venous pulsations are not elevated. Auscultation of the chest reveals clear breath sounds in all lung fields. The surgical incision is intact without erythema or purulent drainage. Inspection of the Foley catheter drainage system reveals that the previously clear, yellow urine has acutely transitioned to a dark, reddish-brown color.

Which of the following is the most appropriate initial step in management?

Stop the transfusion and administer copious intravenous normal saline

Stop the transfusion and administer intravenous furosemide

Stop the transfusion and administer intravenous broad-spectrum antibiotics

Continue the transfusion and administer intravenous acetaminophen and diphenhydramine

Stop the transfusion and transport the patient for computed tomography pulmonary angiography

Show answer & explanation

Answer: Stop the transfusion and administer copious intravenous normal saline

Answer

The most appropriate initial step is to stop the transfusion and administer copious intravenous normal saline.

The correct answer is to stop the transfusion and administer copious intravenous normal saline. This patient's presentation of sudden chills, fever, severe flank pain, hypotension, and dark reddish-brown urine (hemoglobinuria) shortly after starting a blood transfusion is the classic triad for an Acute Hemolytic Transfusion Reaction (AHTR). AHTR is a life-threatening complication most commonly caused by ABO incompatibility resulting from a clerical error. The rapid destruction of donor erythrocytes by preformed host antibodies releases large amounts of free hemoglobin into the plasma. This hemoglobin is filtered by the kidneys, where it can precipitate and cause acute tubular necrosis, leading to oliguric renal failure. The initial, most critical management of AHTR is to immediately halt the offending transfusion to limit further hemolysis, and to provide aggressive volume resuscitation with intravenous normal saline. Copious hydration induces a diuresis that flushes the kidneys, preventing hemoglobin precipitation and mitigating hypotensive shock.

Step-by-Step Solution

Analyze the patient's acute symptom onset in relation to the medical intervention.

The patient developed fever, hypotension, severe flank pain, and gross hemoglobinuria precisely 30 minutes after initiating a packed red blood cell transfusion.

Establishing the temporal relationship of symptoms to the transfusion is critical for identifying a transfusion reaction.

Differentiate the type of transfusion reaction based on the classic clinical triad.

The combination of fever, flank pain, and reddish-brown urine (hemoglobinuria) accompanied by hemodynamic instability is highly specific for an Acute Hemolytic Transfusion Reaction (AHTR).

This specific presentation rules out other complications like TACO (which presents with hypertension and pulmonary edema) and TRALI (which lacks hemoglobinuria and flank pain).

Determine the most critical immediate intervention to prevent morbidity and mortality.

The immediate action is to halt the infusion of incompatible blood and initiate aggressive intravenous hydration with normal saline.

Aggressive volume resuscitation maintains urine output, preventing the precipitation of released hemoglobin in the renal tubules and averting acute tubular necrosis (ATN).

Key Concept

Acute Hemolytic Transfusion Reaction (AHTR) classically presents with fever, flank pain, hemoglobinuria, and hypotension shortly after beginning a blood transfusion; immediate management requires stopping the transfusion and providing aggressive intravenous normal saline to prevent acute tubular necrosis.

Estimated Time:1m 30s

Question 1672Question →

A 35-year-old man with untreated HIV infection is brought to the emergency department by his partner due to a 2-week history of progressively worsening headache, fever, and confusion. His temperature is 38.6°C (101.5°F), blood pressure is 120/78 mmHg, and heart rate is 102/min. Physical examination reveals nuchal rigidity and bilateral sixth cranial nerve palsies. Laboratory studies show a CD4 cell count of 28/mm³. A lumbar puncture is performed, and cerebrospinal fluid (CSF) analysis reveals a leukocyte count of 350/mm³ (85% lymphocytes), glucose of 30 mg/dL, and protein of 150 mg/dL. CSF acid-fast bacilli smear is positive. The patient is admitted to the intensive care unit and started on isoniazid, rifampin, pyrazinamide, ethambutol, and intravenous dexamethasone. Which of the following is the most appropriate timing for initiating antiretroviral therapy in this patient?

Concurrently with the initiation of anti-tuberculous therapy

Within 2 weeks of starting anti-tuberculous therapy

At approximately 4 weeks after starting anti-tuberculous therapy

At approximately 8 weeks after starting anti-tuberculous therapy

After completion of the entire 9- to 12-month anti-tuberculous regimen

Show answer & explanation

Answer: At approximately 8 weeks after starting anti-tuberculous therapy

Answer

At approximately 8 weeks after starting anti-tuberculous therapy

The correct timing is to delay ART for approximately 8 weeks. In patients with HIV who develop tuberculous meningitis, initiating antiretroviral therapy (ART) early leads to a rapid restoration of immune function. This robust inflammatory response directed against mycobacterial antigens in the central nervous system is known as Immune Reconstitution Inflammatory Syndrome (IRIS). In the closed space of the skull, the resulting edema can cause severe neurological deterioration, herniation, and death. Therefore, current guidelines mandate delaying ART for at least 8 weeks in all patients with TB meningitis, regardless of how low their CD4 cell count is.

Step-by-Step Solution

Identify the patient's primary diagnosis and immune status.

The patient has tuberculous (TB) meningitis and advanced HIV with a CD4 count of 28/mm³.

CSF findings (lymphocytic pleocytosis, low glucose, high protein) with a positive AFB smear in an HIV-positive patient confirm TB meningitis. The CD4 count dictates HIV severity.

Determine the standard rules for antiretroviral therapy (ART) initiation in HIV-TB coinfection.

Generally, for pulmonary TB, ART is started within 2 weeks if CD4 < 50/mm³, and within 8 weeks if CD4 ≥ 50/mm³.

Early ART in pulmonary TB reduces HIV-related mortality, though it carries a risk of immune reconstitution inflammatory syndrome (IRIS).

Apply the specific exception for central nervous system (CNS) tuberculosis.

Because the patient has TB meningitis, the standard CD4-based rule is superseded. ART must be delayed for approximately 8 weeks.

CNS-IRIS in the closed space of the cranial vault can cause severe cerebral edema and herniation. Clinical trials have shown that early ART in TB meningitis increases mortality.

Key Concept

In patients with HIV and tuberculous meningitis, antiretroviral therapy (ART) should be delayed for approximately 8 weeks after starting anti-tuberculous therapy, regardless of the CD4 count, to prevent fatal CNS immune reconstitution inflammatory syndrome (IRIS).

Question 1673Question →

A 24-year-old man with a history of type 1 diabetes mellitus is brought to the emergency department with altered mental status, vomiting, and diffuse abdominal pain. His blood pressure is 90/60 mmHg, heart rate is 125/min, and respiratory rate is 28/min with deep, rapid breaths. Laboratory studies reveal a blood glucose of 650 mg/dL, arterial pH of 7.12, serum bicarbonate of 10 mEq/L, and a high anion gap. Urinalysis is strongly positive for ketones. Arrange the following steps in the management of this patient's condition in the most appropriate chronological order.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence is: Administer 0.9% normal saline -> Verify potassium > 3.3 mEq/L -> Initiate IV regular insulin -> Add 5% dextrose to fluids -> Administer subcutaneous basal insulin.

The management of diabetic ketoacidosis follows a strict, step-wise algorithm designed to safely correct volume depletion, hyperglycemia, and ketoacidosis while preventing fatal electrolyte shifts. First, severe fluid deficit is addressed with isotonic 0.9% normal saline. Second, because insulin drives potassium intracellularly, one must verify the serum potassium is greater than 3.3 mEq/L to prevent arrhythmias. Third, an intravenous insulin infusion is started to halt ketone production. Fourth, as blood glucose falls below 200-250 mg/dL, 5% dextrose is added to the fluids so insulin can continue running to close the anion gap without causing hypoglycemia. Finally, once the anion gap is closed, the patient is transitioned to subcutaneous insulin.

Step-by-Step Solution

Restore intravascular volume.

Administer a rapid intravenous bolus of 0.9% normal saline.

Patients with DKA have profound osmotic diuresis and volume depletion. Aggressive fluid resuscitation is the immediate priority to restore perfusion.

Assess for potential electrolyte derangements prior to insulin administration.

Verify serum potassium concentration is > 3.3 mEq/L.

Insulin shifts potassium intracellularly. If given to a hypokalemic patient, it can trigger life-threatening arrhythmias.

Halt ketogenesis and lower blood glucose.

Initiate a continuous intravenous infusion of regular insulin.

Insulin stops lipolysis and ketone body production, correcting the metabolic acidosis.

Prevent hypoglycemia during acidosis correction.

Add 5% dextrose to the maintenance intravenous fluids.

Blood glucose often drops faster than the anion gap closes. Adding dextrose allows insulin to be continued without causing hypoglycemia.

Transition to maintenance therapy.

Administer subcutaneous long-acting basal insulin.

Once the anion gap normalizes and the patient can eat, subcutaneous insulin is administered at least 2 hours before discontinuing the IV insulin drip.

Key Concept

The sequential management of diabetic ketoacidosis (DKA) prioritizes volume resuscitation, followed by potassium assessment, continuous insulin administration, glucose supplementation as levels fall, and finally transition to subcutaneous insulin.

Question 1674Question →

A 19-year-old man with type 1 diabetes mellitus is brought to the emergency department due to altered mental status, generalized weakness, and recurrent vomiting for the past 24 hours. He recently had a respiratory tract infection. Vital signs show a blood pressure of 90/60 mm Hg, heart rate of 124/min, and respiratory rate of 28/min with deep, sighing breaths. Laboratory results show:

Test	Value
Serum glucose	580 mg/dL
Serum sodium	132 mEq/L
Serum potassium	3.1 mEq/L
Serum bicarbonate	10 mEq/L
Arterial pH	7.05

Arrange the following management steps in the most appropriate chronological order from first to last.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct chronological sequence is: 1) Administer intravenous 0.9% normal saline, 2) Administer intravenous potassium chloride, 3) Initiate intravenous regular insulin, 4) Add 5% dextrose to IV fluids, and 5) Transition to subcutaneous insulin.

The correct sequence for DKA management requires addressing the most immediate life-threatening derangements first. Initial resuscitation always begins with isotonic fluids to restore intravascular volume. Next, potassium must be evaluated; because this patient has severe hypokalemia (< 3.3 mEq/L), potassium replacement must precede insulin therapy to avoid lethal arrhythmias. Once potassium is safe (> 3.3 mEq/L), an insulin infusion is started to resolve the ketoacidosis. As glucose levels fall to 200 mg/dL, dextrose is added to the fluids to prevent hypoglycemia while allowing insulin to continue suppressing ketogenesis. Finally, once the anion gap closes and the patient can eat, subcutaneous insulin is initiated.

Step-by-Step Solution

Assess volume status and initiate fluid resuscitation.

The patient is hypotensive and volume depleted. 0.9% normal saline is given rapidly.

Isotonic fluids restore intravascular volume, improve tissue perfusion, and lower counter-regulatory hormones.

Evaluate initial serum potassium level before giving insulin.

Serum potassium is severely low at 3.1 mEq/L.

Insulin drives potassium into cells. If K+ is < 3.3 mEq/L, giving insulin can cause fatal arrhythmias. Potassium must be replenished first.

Initiate insulin therapy once potassium is safe.

Intravenous regular insulin infusion is started.

Insulin is required to suppress ketogenesis and lipolysis, which closes the anion gap and resolves the ketoacidosis.

Monitor blood glucose and adjust fluids.

When glucose drops to 200 mg/dL, 5% dextrose is added to the IV fluids.

Dextrose prevents hypoglycemia while allowing continuous insulin administration until the anion gap closes.

Transition to definitive maintenance therapy.

Subcutaneous basal-bolus insulin is initiated.

Once the anion gap is closed and the patient can tolerate oral intake, the IV insulin infusion can be safely stopped.

Key Concept

In the management of Diabetic Ketoacidosis (DKA), aggressive fluid resuscitation is the first step. If severe hypokalemia (< 3.3 mEq/L) is present, potassium must be repleted before the initiation of insulin to prevent life-threatening cardiac arrhythmias.

Question 1675Question →

A 71-year-old man comes to the clinic for a 4-month history of progressive lower back pain and severe fatigue. He states that the pain is a constant, deep ache that does not improve with rest or over-the-counter ibuprofen. Over the same period, he has developed increasing constipation. He has no prior medical history and takes no daily medications. Vital signs are within normal limits. Physical examination reveals generalized pallor and localized tenderness to palpation over the mid-lumbar spine. Neurologic examination is unremarkable. Laboratory studies show the following:

Test	Result	Reference Range
Hemoglobin	8.9 g/dL	13.5–17.5 g/dL
Mean corpuscular volume	89 fL	80–100 fL
Leukocyte count	6,200/mm³	4,500–11,000/mm³
Platelet count	195,000/mm³	150,000–400,000/mm³
Blood urea nitrogen	34 mg/dL	8–20 mg/dL
Serum creatinine	2.3 mg/dL	0.7–1.3 mg/dL
Serum calcium	11.9 mg/dL	8.4–10.2 mg/dL
Total serum protein	9.1 g/dL	6.0–7.8 g/dL
Serum albumin	3.4 g/dL	3.5–5.5 g/dL

Which of the following is the most appropriate next step in establishing the diagnosis?

Measurement of intact parathyroid hormone

Prostate-specific antigen testing

Radionuclide bone scan

Renal ultrasonography

Serum and urine protein electrophoresis

Show answer & explanation

Answer: Serum and urine protein electrophoresis

Answer

Serum and urine protein electrophoresis

The correct answer is serum and urine protein electrophoresis. This patient's presentation of normocytic anemia, renal insufficiency, hypercalcemia, and bone pain is highly suspicious for multiple myeloma (often remembered by the acronym CRAB). A critical laboratory clue is the elevated protein gap (total serum protein minus serum albumin =

9.1 - 3.4 = 5.7

g/dL). A gap greater than

4.0

g/dL indicates an excess of non-albumin proteins in the blood, which are typically immunoglobulins. The most appropriate initial step to evaluate for a monoclonal gammopathy is serum and urine protein electrophoresis (SPEP/UPEP), which can detect a monoclonal protein spike (M-spike) produced by the neoplastic plasma cells.

Step-by-Step Solution

Identify the patient's core clinical syndrome.

The patient presents with bone pain, normocytic anemia, hypercalcemia, and renal insufficiency.

Recognizing this constellation of symptoms is essential for forming an accurate differential diagnosis.

Analyze the laboratory findings for clues to the underlying etiology.

There is a significantly elevated total protein with a normal-to-low albumin, indicating an elevated 'protein gap' (Total Protein - Albumin

> 4

g/dL).

A large protein gap suggests the presence of an excess of non-albumin proteins in the blood, typically immunoglobulins, pointing toward a monoclonal gammopathy.

Determine the most appropriate diagnostic test for the suspected condition.

The combination of CRAB symptoms (Hypercalcemia, Renal failure, Anemia, Bone pain) and a protein gap strongly suggests multiple myeloma. This is diagnosed initially via serum and urine protein electrophoresis.

Protein electrophoresis detects the monoclonal antibody spike (M-spike) characteristic of multiple myeloma, confirming the underlying plasma cell dyscrasia.

Key Concept

Multiple myeloma classically presents with hypercalcemia, renal insufficiency, anemia, and bone pain (CRAB). An elevated protein gap (total protein minus albumin

> 4

g/dL) is a strong clue for a monoclonal gammopathy, and the essential initial diagnostic step is serum and urine protein electrophoresis.

Question 1676Question →

A 54-year-old woman is evaluated in the oncology ward for a new-onset fever. She is currently on day 10 of induction chemotherapy for acute myeloid leukemia. Her temperature is 38.8°C (101.8°F), blood pressure is 105/65 mmHg, and heart rate is 112/min. Laboratory analysis shows a white blood cell count of 400/mm³ with an absolute neutrophil count (ANC) of 150/mm³. She has a central venous port in her right internal jugular vein.

Arrange the following interventions in the most appropriate chronological sequence for managing this patient's condition.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence for managing neutropenic fever is to first obtain appropriate blood cultures, immediately follow with empiric antipseudomonal antibiotics, later tailor the regimen based on culture results, and finally add empiric antifungal therapy if the fever persists beyond 4 to 7 days.

The standard protocol for neutropenic fever requires an immediate, sequential approach. Blood cultures must be drawn first to identify the organism, followed urgently by empiric antipseudomonal antibiotics to prevent septic shock. Once culture results return (usually 48-72 hours later), the antibiotics are tailored. Finally, if the patient remains febrile and neutropenic after 4 to 7 days, empiric antifungal therapy is added.

Step-by-Step Solution

Obtain blood cultures from the central venous port and a peripheral vein.

Samples are sent for microbiologic analysis.

Blood cultures must be drawn before starting antibiotics to prevent sterilization of the sample and allow for the identification of the causative pathogen.

Administer empiric intravenous antipseudomonal therapy (e.g., cefepime).

The patient receives broad-spectrum antibacterial coverage.

Neutropenic patients lack the immune response to control infections; delaying empiric antibiotics significantly increases the risk of mortality.

Tailor the antibacterial regimen based on finalized culture and susceptibility results.

Antibiotics are de-escalated or adjusted.

Once the pathogen and its susceptibilities are known, targeted therapy reduces the risk of resistance and medication toxicity, adhering to principles of antimicrobial stewardship.

Initiate empiric intravenous antifungal therapy if the patient remains febrile after 4 to 7 days.

The patient receives coverage for invasive fungal pathogens.

Persistent fever in a profoundly neutropenic patient despite broad-spectrum antibiotics suggests a high likelihood of an underlying fungal infection, most commonly Candida or Aspergillus species.

Key Concept

The chronological management algorithm for neutropenic fever prioritizes immediate cultures and empiric antipseudomonal antibiotics, followed by culture-directed tailoring, and reserves empiric antifungals for those who remain febrile after 4 to 7 days.

Estimated Time:1m 0s

Question 1677Question →

A pulmonology consultation is requested for a hospitalized patient who was admitted two days ago for an acute exacerbation of chronic obstructive pulmonary disease. The patient has a history of type 2 diabetes mellitus and has developed severe nausea and intractable emesis over the last 24 hours after starting a new medication regimen. Current vital signs show a temperature of $37.8^\circ C$ ( $100^\circ F$ ), blood pressure of $110/68$ mm Hg, pulse of $115$ /min, and respirations of $28$ /min. Physical examination reveals distant breath sounds with prolonged expiration and dry mucous membranes. A comprehensive metabolic panel and arterial blood gas analysis are obtained on room air.

Laboratory Test	Result	Reference Range
Sodium	140 mEq/L	136-145 mEq/L
Potassium	3.2 mEq/L	3.5-5.0 mEq/L
Chloride	84 mEq/L	98-106 mEq/L
Bicarbonate	18 mEq/L	22-28 mEq/L
Blood urea nitrogen	45 mg/dL	8-20 mg/dL
Creatinine	2.1 mg/dL	0.7-1.3 mg/dL
Glucose	480 mg/dL	70-110 mg/dL
Arterial pH	7.26	7.35-7.45
Arterial $P_{a}CO_2$	44 mm Hg	35-45 mm Hg
Arterial $P_{a}O_2$	60 mm Hg	75-105 mm Hg

Based on the laboratory data provided, which of the following accurately describes the primary and concurrent acid-base processes occurring in this patient?

High anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosis

High anion gap metabolic acidosis and respiratory acidosis only

High anion gap metabolic acidosis and metabolic alkalosis only

High anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosis

Show answer & explanation

Answer: High anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosis

Answer

High anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosis

The correct answer accurately identifies a triple acid-base disorder through a four-step calculation. First, the acidemia (

pH = 7.26

) and low bicarbonate (

18

mEq/L) indicate a primary metabolic acidosis. Second, the anion gap (

140 - (84 + 18) = 38

) is severely elevated, confirming a high anion gap metabolic acidosis. Third, Winter's formula (

1.5 \times 18 + 8 = 35

) demonstrates that the patient's measured

P_{a}CO_2

(

44

mm Hg) is significantly higher than the expected compensatory range (

33-37

mm Hg), revealing a concurrent respiratory acidosis. Finally, the delta-delta calculation (Corrected

[HCO_3^-] = \text{Measured } [HCO_3^-] + (\text{Measured AG} - 12) = 18 + (38 - 12) = 44

) shows a corrected bicarbonate much higher than the normal

24

mEq/L, confirming a superimposed metabolic alkalosis.

Step-by-Step Solution

Determine the primary disorder from arterial pH and serum bicarbonate.

Primary metabolic acidosis.

The arterial pH is 7.26 (acidemia), which corresponds with the low bicarbonate level of 18 mEq/L.

Calculate the serum anion gap to classify the metabolic acidosis.

Anion gap is 38 mEq/L, indicating High Anion Gap Metabolic Acidosis (HAGMA).

Anion Gap =

Na^+ - (Cl^- + HCO_3^-)

= 140 - (84 + 18) = 38. A normal gap is

12 \pm 4

. The elevated gap suggests accumulation of unmeasured acids (likely diabetic ketoacidosis given the glucose of 480 mg/dL).

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected

P_{a}CO_2

35 \pm 2

mm Hg; actual is 44 mm Hg, indicating a concurrent respiratory acidosis.

Winter's formula: Expected

P_{a}CO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 18 + 8 \pm 2 = 35 \pm 2

. Because the actual

P_{a}CO_2

(44) is greater than the expected range (33-37), there is a primary respiratory acidosis (due to COPD exacerbation).

Calculate the delta gap and corrected bicarbonate to check for a third disorder.

Corrected bicarbonate is 44 mEq/L, indicating a concurrent metabolic alkalosis.

\Delta AG

= Measured AG (38) - Normal AG (12) = 26. Corrected

[HCO_3^-] = \Delta AG +

Measured

[HCO_3^-] = 26 + 18 = 44

. Since 44 is significantly greater than the normal baseline of 24, a concurrent metabolic alkalosis is present (due to emesis).

Key Concept

Diagnosis of mixed acid-base disorders requires a sequential, stepwise calculation approach utilizing the anion gap, Winter's formula, and the delta gap.

Question 1678Question →

A 65-year-old woman is evaluated in the clinic for progressive fatigue and generalized weakness over the past month. Her past medical history is significant for type 2 diabetes mellitus, essential hypertension, and heart failure with preserved ejection fraction. Two months ago, a new medication was added to her regimen to improve fluid balance and blood pressure control. Her current medications include metformin, amlodipine, empagliflozin, and the newly prescribed drug.

Her temperature is 37.0°C (98.6°F), blood pressure is 128/78 mm Hg, pulse is 72/min, and respirations are 16/min. Physical examination reveals clear lungs and no lower extremity edema.

Laboratory studies are obtained:

Test	Value
Serum sodium	$138 \text{ mEq/L}$
Serum potassium	$5.8 \text{ mEq/L}$
Serum chloride	$109 \text{ mEq/L}$
Serum bicarbonate	$17 \text{ mEq/L}$
Blood urea nitrogen	$28 \text{ mg/dL}$
Serum creatinine	$1.6 \text{ mg/dL}$
Serum glucose	$145 \text{ mg/dL}$

An arterial blood gas on room air shows:

Test	Value
pH	$7.33$
PaCO $_2$	$33 \text{ mm Hg}$
PaO $_2$	$92 \text{ mm Hg}$

Which of the following best explains the underlying pathophysiology of this patient's current acid-base and electrolyte abnormalities?

Decreased apical sodium channel activity in the cortical collecting duct

Defective hydrogen ion secretion by alpha-intercalated cells in the distal tubule

Impaired carbonic anhydrase-dependent bicarbonate reabsorption in the proximal tubule

Concurrent primary respiratory alkalosis driven by an exaggerated ventilatory response

Accumulation of unmeasured endogenous organic anions in the serum

Show answer & explanation

Answer: Decreased apical sodium channel activity in the cortical collecting duct

Answer

The correct answer is decreased apical sodium channel activity in the cortical collecting duct.

The correct answer accurately describes the pathophysiology of Type 4 renal tubular acidosis (RTA). The patient presents with a primary metabolic acidosis (pH 7.33, HCO3 17), a normal anion gap (138 - 126 = 12), appropriate respiratory compensation (Winter's formula:

1.5 \times 17 + 8 = 33.5

), and hyperkalemia (K+ 5.8). This triad of NAGMA, hyperkalemia, and mild renal impairment in a diabetic patient on a newly added heart failure medication (e.g., spironolactone or an ACE inhibitor) strongly points to hypoaldosteronism or aldosterone resistance (Type 4 RTA). Aldosterone normally stimulates the epithelial sodium channel (ENaC) in the principal cells of the cortical collecting duct. Decreased ENaC activity reduces sodium reabsorption, impairing the generation of the lumen-negative transepithelial voltage. This loss of electrical gradient prevents the normal secretion of potassium and hydrogen ions, resulting in hyperkalemia and metabolic acidosis.

Step-by-Step Solution

Analyze the primary acid-base disturbance using the arterial blood gas and serum chemistry.

The pH of 7.33 indicates acidemia, and the low serum bicarbonate (17 mEq/L) confirms a primary metabolic acidosis.

Identifying the primary derangement is the first step in approaching any acid-base disorder.

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap = Na - (Cl + HCO3) = 138 - (109 + 17) = 12 mEq/L. This is a normal anion gap metabolic acidosis (NAGMA).

Differentiating between normal and high anion gap narrows down the differential diagnosis significantly.

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected PaCO2 = 1.5(HCO3) + 8 = 1.5(17) + 8 = 33.5 ± 2 mm Hg. The actual PaCO2 is 33 mm Hg, indicating perfect respiratory compensation with no concurrent respiratory disorder.

This rules out a mixed acid-base disorder.

Evaluate the serum potassium level in the context of the NAGMA.

The patient has hyperkalemia (5.8 mEq/L) alongside the NAGMA.

A hyperkalemic normal anion gap metabolic acidosis is the hallmark of Type 4 renal tubular acidosis (RTA).

Identify the cellular mechanism causing Type 4 RTA in this clinical vignette.

The patient recently started a heart failure medication (likely spironolactone, a mineralocorticoid receptor antagonist). This blocks aldosterone action, leading to decreased epithelial sodium channel (ENaC) activity in the cortical collecting duct.

Reduced ENaC activity diminishes the lumen-negative potential, which is the driving force for both potassium and hydrogen ion secretion, culminating in hyperkalemia and acidosis.

Key Concept

Type 4 Renal Tubular Acidosis (Hyperkalemic RTA)

Estimated Time:1m 30s

Question 1679Question →

A $52$ -year-old man presents for a routine follow-up appointment due to progressive fatigue, a $4\text{-kg}$ ( $8.8\text{-lb}$ ) weight gain, and an inability to tolerate cold environments over the past $5$ months. Three years ago, he underwent surgical resection of a large pituitary adenoma. His temperature is $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ), blood pressure is $115/75\text{ mmHg}$ , and pulse is $56/\text{min}$ . Physical examination reveals dry skin and delayed relaxation of the deep tendon reflexes. Laboratory testing shows:

Serum free $\text{T}_4$ : $0.4\text{ ng/dL}$ (normal: $0.9-1.7\text{ ng/dL}$ )
Serum $\text{TSH}$ : $1.1\text{ }\mu\text{U/mL}$ (normal: $0.5-5.0\text{ }\mu\text{U/mL}$ )

Which of the following is the most likely diagnosis?

Primary hypothyroidism

Subclinical hypothyroidism

Central hypothyroidism

Euthyroid sick syndrome

Subacute thyroiditis

Show answer & explanation

Answer: Central hypothyroidism

Answer

Central hypothyroidism

This patient exhibits classic signs and symptoms of hypothyroidism. In a normally functioning hypothalamic-pituitary-thyroid axis, a low free

\text{T}_4

level would stimulate the anterior pituitary to secrete high levels of

\text{TSH}

. This patient's normal

\text{TSH}

level is inappropriate for his low

\text{T}_4

, indicating a failure of the pituitary gland to respond. Given his history of pituitary adenoma resection, this points directly to central (secondary) hypothyroidism.

Step-by-Step Solution

Identify the patient's clinical syndrome.

The patient has signs and symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, delayed reflexes) along with a history of pituitary adenoma resection.

To establish the primary clinical problem and potential underlying causes.

Analyze the thyroid function tests.

The free T4 level is abnormally low, confirming the hypothyroid state. However, the TSH level is within the normal range.

To classify the hypothyroidism based on the hypothalamic-pituitary-thyroid axis feedback loop.

Synthesize findings to determine the etiology.

A low free T4 with an inappropriately normal TSH in a patient with previous pituitary surgery confirms a diagnosis of central hypothyroidism.

To arrive at the final diagnosis.

Key Concept

Interpretation of thyroid function tests to differentiate primary from central hypothyroidism.

Question 1680Question →

A 71-year-old man presents to the emergency department with a 4-day history of productive cough, chills, and progressive dyspnea. On physical examination, he appears lethargic and is using accessory respiratory muscles. Vital signs are:

Temperature: 38.8°C (101.8°F)
Blood pressure: 92/58 mm Hg
Heart rate: 115/min
Respiratory rate: 28/min

Arterial blood gas analysis on room air reveals:
pH: 7.18
pCO2: 38 mm Hg
pO2: 54 mm Hg

Serum laboratory studies reveal:
Sodium: 139 mEq/L
Potassium: 4.6 mEq/L
Chloride: 101 mEq/L
Bicarbonate: 14 mEq/L
Blood urea nitrogen: 32 mg/dL
Creatinine: 1.6 mg/dL
Lactate: 5.4 mmol/L

Based on the patient's laboratory findings, which of the following is the most accurate description of his current acid-base status?

Anion gap metabolic acidosis with superimposed respiratory acidosis

Anion gap metabolic acidosis with appropriate respiratory compensation

Anion gap metabolic acidosis with superimposed respiratory alkalosis

Primary respiratory acidosis with appropriate metabolic compensation

Non-anion gap metabolic acidosis with appropriate respiratory compensation

Show answer & explanation

Answer: Anion gap metabolic acidosis with superimposed respiratory acidosis

Answer

Anion gap metabolic acidosis with superimposed respiratory acidosis

The patient's laboratory results indicate an initial acid-base disturbance of metabolic acidosis, evidenced by a low pH (7.18) and a low bicarbonate level (14 mEq/L). The next step is to calculate the anion gap, which is

Na^+ - (Cl^- + HCO_3^-) = 139 - (101 + 14) = 24

. Since this is greater than the normal range of 8-12, the patient has an anion gap metabolic acidosis (likely secondary to elevated lactate from sepsis/hypoperfusion). Finally, Winter's formula should be used to determine if the respiratory compensation is appropriate: Expected

pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2

. For this patient, Expected

pCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2

mm Hg. The patient's measured

pCO_2

is 38 mm Hg, which is significantly higher than the expected range (27-31 mm Hg). A measured

pCO_2

higher than the expected value indicates a concurrent, or superimposed, respiratory acidosis. In this clinical context, the elevated

pCO_2

reflects impending respiratory failure due to respiratory muscle fatigue from severe pneumonia.

Step-by-Step Solution

Identify the primary acid-base disorder by evaluating the pH and serum bicarbonate.

The pH is 7.18 (acidemia) and the bicarbonate is 14 mEq/L, indicating a primary metabolic acidosis.

Determining the primary disorder establishes the baseline for all subsequent calculations.

Calculate the anion gap using the formula:

Na^+ - (Cl^- + HCO_3^-)

The anion gap is

139 - (101 + 14) = 24

. Since this is greater than 12, there is an anion gap metabolic acidosis.

Classifying the metabolic acidosis as anion gap or non-anion gap identifies the etiology (in this case, lactic acidosis from severe pneumonia/sepsis).

Apply Winter's formula to determine the expected

pCO_2

for appropriate respiratory compensation: Expected

pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2

Expected

pCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2

mm Hg (Range: 27 to 31 mm Hg).

Comparing expected

pCO_2

to the measured

pCO_2

reveals whether a secondary respiratory acid-base disorder is concurrently present.

Compare the patient's measured

pCO_2

to the expected compensatory range.

The measured

pCO_2

is 38 mm Hg, which is higher than the maximum expected value of 31 mm Hg.

An inappropriately elevated

pCO_2

in the setting of severe metabolic acidosis indicates inadequate ventilation (superimposed respiratory acidosis), pointing to respiratory muscle fatigue.

Key Concept

In patients with metabolic acidosis, expected respiratory compensation must be calculated using Winter's formula. An observed

pCO_2

higher than the expected range indicates a superimposed respiratory acidosis.

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