Internal Medicine

3151 questions

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A $24$ -year-old man with a history of type 1 diabetes mellitus is brought to the emergency department with abdominal pain, vomiting, and confusion. Laboratory evaluation reveals the following:

Laboratory Test	Result	Reference Range
Serum glucose	$580 \text{ mg/dL}$	$70-99 \text{ mg/dL}$
Arterial pH	$7.15$	$7.35-7.45$
Serum bicarbonate	$9 \text{ mEq/L}$	$22-28 \text{ mEq/L}$
Serum potassium	$3.0 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Anion gap	$26 \text{ mEq/L}$	$8-12 \text{ mEq/L}$

He is diagnosed with severe diabetic ketoacidosis (DKA). Arrange the following management steps in the most appropriate chronological sequence for this patient.

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Answer

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

The standard management sequence for DKA prioritizes ABCs and fluid resuscitation. Therefore, IV normal saline is always first. The next critical decision point is the potassium level. Because this patient's potassium is dangerously low at

3.0 \text{ mEq/L}

, potassium replacement must occur before insulin is given. Once potassium is

\geq 3.3 \text{ mEq/L}

, continuous IV insulin is started. When blood glucose drops to approximately

200 \text{ mg/dL}

5\%

dextrose is added to the IV fluids to allow continued insulin administration for ketone clearance without causing hypoglycemia. Finally, after the anion gap is closed (

\leq 12 \text{ mEq/L}

) and the patient can eat, IV insulin is transitioned to a subcutaneous regimen.

Step-by-Step Solution

Assess volume status and initiate immediate fluid resuscitation.

Administration of

0.9\%

sodium chloride.

The patient is profoundly dehydrated due to osmotic diuresis; volume expansion is always the immediate first step in DKA management.

Evaluate the serum potassium level before administering insulin.

Administration of intravenous potassium chloride.

The patient has severe hypokalemia (

3.0 \text{ mEq/L}

). Insulin drives potassium intracellularly; administering insulin now would precipitate fatal cardiac arrhythmias. Potassium must be replaced first.

Begin insulin therapy once the potassium level is safe (

\geq 3.3 \text{ mEq/L}

Initiation of intravenous regular insulin infusion.

Insulin halts ketogenesis and lipolysis, correcting the underlying acidosis and lowering blood glucose.

Monitor blood glucose and adjust intravenous fluids accordingly.

Addition of

5\%

dextrose to the IV fluids once glucose drops to

\leq 200 \text{ mg/dL}

This prevents hypoglycemia. The insulin drip must remain on to clear the ketones and close the anion gap.

Transition to maintenance therapy.

Transition to a subcutaneous insulin regimen.

This is done once the anion gap closes, acidosis resolves, and the patient can tolerate an oral diet.

Key Concept

In the management of diabetic ketoacidosis (DKA), profound hypokalemia (

< 3.3 \text{ mEq/L}

) requires immediate potassium replacement prior to the initiation of insulin therapy to prevent fatal cardiac arrhythmias.

Question 1582Question →

A $42$ -year-old male is evaluated in the urgent care clinic for persistent nausea and non-bilious emesis that has occurred multiple times daily over the past four days. He reports feeling profoundly weak and dizzy when standing up. His blood pressure is $98/60$ mm Hg while supine and drops to $82/50$ mm Hg upon standing. Heart rate is $112$ /min. Physical examination reveals dry oral mucosa and decreased skin turgor. Laboratory studies show:

$Na^+$ : $137$ mEq/L
$K^+$ : $2.8$ mEq/L
$Cl^-$ : $86$ mEq/L
$HCO_3^-$ : $36$ mEq/L
$BUN$ : $42$ mg/dL
Creatinine: $1.3$ mg/dL

Arterial blood gas reveals a $pH$ of $7.53$ and a $pCO_2$ of $46$ mm Hg.

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

Intravenous

5\%

dextrose in water

Intravenous acetazolamide

Intravenous

0.9\%

sodium chloride

Intravenous sodium bicarbonate

Oral spironolactone

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Answer: Intravenous $0.9\%$ sodium chloride

Answer

Intravenous

0.9\%

sodium chloride

The correct intervention is administering intravenous isotonic normal saline. This patient has a classic saline-responsive metabolic alkalosis secondary to severe and recurrent vomiting. The profound loss of gastric fluid leads to a hypochloremic, hypokalemic metabolic alkalosis complicated by severe intravascular volume depletion (evidenced by his orthostatic hypotension, tachycardia, and elevated BUN-to-creatinine ratio). The body's response to hypovolemia is to activate the renin-angiotensin-aldosterone system. Aldosterone acts on the distal nephron to aggressively reabsorb sodium to maintain blood pressure, but this occurs at the obligate expense of excreting potassium and hydrogen ions. Therefore, the kidneys cannot excrete the excess bicarbonate until the volume and chloride deficits are corrected. Intravenous

0.9\%

sodium chloride restores the intravascular volume and provides the necessary chloride, which deactivates the aldosterone surge and allows the kidneys to excrete bicarbonate, naturally resolving the alkalosis.

Step-by-Step Solution

Assess the patient's volume status and acid-base derangement from the clinical presentation and laboratory results.

The patient has marked orthostatic hypotension and dry mucosa indicating severe volume depletion, alongside a hypokalemic, hypochloremic metabolic alkalosis.

Recurrent emesis causes significant loss of gastric fluid, which is rich in hydrochloric acid and potassium.

Determine the pathophysiological mechanism maintaining the alkalosis.

Volume and chloride depletion trigger the activation of the renin-angiotensin-aldosterone system.

Aldosterone promotes aggressive renal sodium reabsorption at the expense of potassium and hydrogen ion excretion, meaning the kidneys prioritize volume preservation over correcting the acid-base balance.

Select the appropriate therapy to break the maintenance cycle.

Administration of isotonic normal saline.

Restoring intravascular volume with a chloride-rich fluid deactivates aldosterone secretion, which allows the kidneys to safely excrete the excess bicarbonate and correct the alkalosis.

Key Concept

Saline-responsive metabolic alkalosis caused by vomiting is maintained by volume depletion and hypochloremia, and is treated initially with intravenous 0.9% sodium chloride.

Question 1583Question →

An unresponsive $59$ -year-old man is transported to the emergency department by ambulance after his partner noticed profound confusion, severe weakness, and abdominal pain over the past $48$ hours. He has a known history of locally advanced head and neck squamous cell carcinoma. According to his partner, he has been drinking excessive amounts of water and urinating frequently for the past week, and has not had a bowel movement in $5$ days. Vital signs reveal a heart rate of $115/\text{min}$ and blood pressure of $98/60\text{ mmHg}$ . The patient is afebrile. Physical examination demonstrates notably dry mucous membranes, delayed capillary refill, and generalized hypotonia.

Laboratory evaluation shows:
Serum sodium: $139\text{ mEq/L}$
Serum potassium: $4.1\text{ mEq/L}$
Serum chloride: $104\text{ mEq/L}$
Serum bicarbonate: $25\text{ mEq/L}$
Blood urea nitrogen: $52\text{ mg/dL}$
Serum creatinine: $2.3\text{ mg/dL}$
Serum total calcium: $12.8\text{ mg/dL}$
Serum albumin: $1.5\text{ g/dL}$
Serum phosphorus: $2.0\text{ mg/dL}$

An electrocardiogram demonstrates sinus tachycardia with a shortened QT interval.

What is the most appropriate initial step in the management of this patient?

Intravenous normal saline infusion

Intravenous furosemide administration

Intravenous zoledronic acid administration

Emergent hemodialysis

Intravenous broad-spectrum antibiotic therapy

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Answer: Intravenous normal saline infusion

Answer

Intravenous normal saline infusion

The correct answer is intravenous normal saline infusion. This patient has a corrected serum calcium of

14.8\text{ mg/dL}

(calculated as

\text{Corrected Ca} = 12.8 + 0.8 \times [4.0 - 1.5]

). Severe hypercalcemia (defined as

>14\text{ mg/dL}

) frequently causes a nephrogenic diabetes insipidus-like state by downregulating aquaporin channels in the collecting ducts, resulting in profound polyuria, severe volume depletion, and prerenal acute kidney injury. The initial and most critical step in managing severe hypercalcemia is aggressive volume expansion with isotonic normal saline. This intervention achieves three vital goals: it rapidly restores intravascular volume, it improves the glomerular filtration rate, and it directly enhances the renal excretion of calcium by decreasing proximal tubular calcium reabsorption.

Step-by-Step Solution

Calculate the corrected serum calcium to accurately assess the severity of hypercalcemia in the setting of hypoalbuminemia.

Using the formula

\text{Corrected Ca} = \text{Total Ca} + 0.8 \times (4.0 - \text{Albumin})

, the value is

12.8 + 0.8 \times (4.0 - 1.5) = 14.8\text{ mg/dL}

Serum calcium is highly protein-bound, primarily to albumin. A low albumin level artificially lowers the measured total calcium, masking the true extent of biologically active ionized calcium.

Identify the pathophysiologic consequences of severe hypercalcemia based on the clinical presentation.

The patient's corrected calcium is

>14\text{ mg/dL}

, which triggers a nephrogenic diabetes insipidus-like state, leading to massive polyuria, volume depletion, and prerenal acute kidney injury.

The clinical presentation of thirst, polyuria, hypotension, and an elevated

\text{BUN}/\text{Creatinine}

ratio confirms severe intravascular volume depletion.

Determine the most appropriate initial pharmacological intervention for hypercalcemic crisis.

The absolute first step is aggressive intravenous volume expansion with isotonic normal saline.

Normal saline rapidly restores intravascular volume, improves the glomerular filtration rate, and significantly enhances the renal excretion of calcium.

Key Concept

The initial management of severe hypercalcemia (corrected calcium

>14\text{ mg/dL}

) requires aggressive intravenous fluid resuscitation with normal saline to correct the profound volume depletion caused by calcium-induced nephrogenic diabetes insipidus.

Question 1584Question →

An internal medicine resident is reviewing the electronic medical record of a $50$ -year-old female patient who recently underwent routine laboratory screening. The patient had mentioned experiencing mild, intermittent fatigue over the last few months but otherwise felt well. She has no significant past medical history and takes no medications. The physical examination documented in the chart is entirely unremarkable, with a normal body mass index and a resting heart rate of $72/\text{min}$ . Her thyroid panel reveals a serum thyroid-stimulating hormone (TSH) level of $8.2\text{ mU/L}$ (normal: $0.5 - 5.0\text{ mU/L}$ ) and a serum free thyroxine (T4) level of $1.3\text{ ng/dL}$ (normal: $0.9 - 1.7\text{ ng/dL}$ ). Which of the following is the most accurate diagnosis based on these laboratory findings?

Subclinical hypothyroidism

Overt primary hypothyroidism

Central hypothyroidism

Euthyroid sick syndrome

Thyroid hormone resistance syndrome

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Answer: Subclinical hypothyroidism

Answer

Subclinical hypothyroidism

The correct answer is subclinical hypothyroidism, which is biochemically defined by an elevated serum TSH level in the presence of a normal serum free T4 level. Patients are often asymptomatic or have only mild, nonspecific symptoms such as fatigue.

Step-by-Step Solution

Interpret the serum TSH level.

The TSH is elevated at

8.2\text{ mU/L}

TSH is the most sensitive initial indicator of primary thyroid dysfunction.

Interpret the serum free T4 level in the context of the elevated TSH.

The free T4 is within normal limits at

1.3\text{ ng/dL}

The free T4 level distinguishes between subclinical and overt primary thyroid disease.

Synthesize the laboratory findings to determine the diagnosis.

The combination of an elevated TSH and a normal free T4 indicates subclinical hypothyroidism.

Overt primary hypothyroidism would present with an elevated TSH and a low free T4, rather than a normal one.

Key Concept

Subclinical hypothyroidism is defined biochemically by an elevated serum TSH level in the presence of a normal serum free T4 level.

Question 1585Question →

A 28-year-old man with a history of HLA-B27 positivity is diagnosed with active ankylosing spondylitis. He presents with worsening inflammatory back pain and morning stiffness. Arrange the following therapeutic steps in the standard chronological sequence of escalation for the management of his condition, assuming an inadequate response or disease progression at each preceding stage.

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Answer

The correct sequence of therapy escalation for ankylosing spondylitis begins with continuous NSAIDs, followed by a TNF inhibitor, then an IL-17 inhibitor if the TNF inhibitor fails, and finally surgical intervention for irreversible structural damage.

The established algorithm for axial spondyloarthritis (ankylosing spondylitis) mandates starting with continuous NSAIDs. If disease activity remains high, biological therapy is initiated, typically starting with a TNF inhibitor. If this fails, transitioning to a different biologic class (such as an IL-17 inhibitor) is indicated. Finally, if all medical management fails to prevent severe structural damage leading to profound functional impairment, surgical options like spinal osteotomy are explored.

Step-by-Step Solution

Identify the first-line treatment for active ankylosing spondylitis.

Continuous NSAIDs and physical therapy are selected.

Unlike rheumatoid arthritis where conventional DMARDs are first-line, axial spondyloarthritis responds best initially to continuous NSAID therapy.

Determine the next step for a patient who fails continuous NSAID therapy.

A biologic DMARD, specifically a TNF inhibitor, is added.

TNF inhibitors have robust efficacy in reducing axial inflammation and are the preferred first-line biologics for this condition.

Determine the next pharmacologic step if a TNF inhibitor fails.

Therapy is switched to an IL-17 inhibitor.

When a first biologic fails, guidelines recommend switching classes. IL-17 inhibitors (e.g., secukinumab, ixekizumab) are highly effective alternative options for axial disease.

Identify the end-stage management for refractory structural complications.

Surgical evaluation is considered.

Medical therapy cannot reverse fixed bony ankylosis; severe functional impairment from structural damage may require osteotomy or arthroplasty.

Key Concept

The stepwise pharmacological and procedural escalation in ankylosing spondylitis progresses from continuous NSAIDs to TNF inhibitors, followed by IL-17 inhibitors for non-responders, and ultimately surgery for severe structural deformities.

Question 1586Question →

A $24$ -year-old man with a history of type $1$ diabetes mellitus is brought to the emergency department due to altered mental status, nausea, and vomiting for the past $24$ hours. He recently had an upper respiratory tract infection. Vital signs are temperature $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure $90/60\text{ mm Hg}$ , pulse $120\text{/min}$ , and respirations $26\text{/min}$ with deep, rapid breaths. Laboratory studies show a serum glucose of $580 \text{ mg/dL}$ , arterial pH of $7.12$ , serum bicarbonate of $10 \text{ mEq/L}$ , and an anion gap of $22 \text{ mEq/L}$ . Urine is strongly positive for ketones. Arrange the following interventions in the most appropriate chronological order for the acute management of this patient.

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Answer

The correct chronological sequence for Diabetic Ketoacidosis management is: 1) Administer intravenous normal saline, 2) Verify serum potassium levels, 3) Initiate intravenous regular insulin, 4) Add 5% dextrose when glucose drops below 200 mg/dL, and 5) Transition to subcutaneous insulin when the anion gap closes.

Optimal management of Diabetic Ketoacidosis follows a strict evidence-based sequence. Initial rapid fluid resuscitation with normal saline restores intravascular volume and renal perfusion. Next, potassium must be assessed (and replaced if necessary) before starting insulin to avoid life-threatening hypokalemia. Only after confirming a safe potassium level (

> 3.3 \text{ mEq/L}

) is an intravenous insulin drip started to correct the ketoacidosis. As glucose levels predictably fall below

200 \text{ mg/dL}

, dextrose is added to the fluids to prevent hypoglycemia while allowing continued insulin administration to successfully close the anion gap. Finally, once the anion gap normalizes and the patient can eat, they are safely transitioned to a subcutaneous insulin regimen.

Step-by-Step Solution

Restore intravascular volume

Administer a bolus of intravenous

0.9\%

normal saline

Corrects profound dehydration and improves renal perfusion, which is necessary for the initial clearance of serum glucose and ketones.

Assess electrolyte safety

Verify serum potassium is

> 3.3 \text{ mEq/L}

Insulin shifts potassium into cells. Administering insulin to a patient with severe hypokalemia can induce fatal cardiac arrhythmias.

Correct hyperglycemia and ketoacidosis

Start continuous intravenous regular insulin infusion

Insulin halts lipolysis and ketone body production, directly treating the underlying pathophysiology of Diabetic Ketoacidosis.

Prevent iatrogenic hypoglycemia

Add

5\%

dextrose to intravenous fluids when glucose is

< 200 \text{ mg/dL}

Insulin must be continued until the anion gap closes to resolve the ketoacidosis. Adding dextrose prevents hypoglycemia during this ongoing treatment.

Transition to maintenance therapy

Administer subcutaneous insulin and overlap with the intravenous infusion

Once Diabetic Ketoacidosis resolves (indicated by a closed anion gap) and the patient can eat, subcutaneous insulin is started to provide long-acting coverage before stopping the intravenous drip.

Key Concept

The stepwise management of Diabetic Ketoacidosis (DKA)

Question 1587Question →

Match each patient's clinical presentation with the most characteristic peripheral blood smear finding associated with their underlying hematologic condition.

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Answer

The clinical presentations correctly match as follows: the patient with sulfa-induced hemolysis (G6PD deficiency) matches with bite cells; the CLL patient with positive DAT (autoimmune hemolytic anemia) matches with spherocytes; the patient with lifelong microcytosis (thalassemia trait) matches with target cells; and the patient with a dry tap and splenomegaly (primary myelofibrosis) matches with teardrop cells.

Each correct pairing relies on recognizing the classic clinical vignette for a specific hematologic disorder and linking it to its pathophysiological morphological hallmark. G6PD deficiency causes oxidative damage (bite cells), autoimmune hemolytic anemia causes macrophage-mediated membrane loss (spherocytes), thalassemia alters the surface-to-volume ratio (target cells), and primary myelofibrosis mechanically squeezes red cells (teardrop cells).

Step-by-Step Solution

Diagnose the 24-year-old man with acute hemolysis following sulfa drug exposure as having G6PD deficiency.

G6PD deficiency is identified.

Oxidative stress in G6PD deficiency causes hemoglobin denaturation (Heinz bodies) which splenic macrophages bite out, forming bite cells.

Identify the cause of hemolysis in the 65-year-old woman with CLL and a positive direct antiglobulin test.

Warm autoimmune hemolytic anemia (AIHA) is diagnosed.

AIHA leads to partial phagocytosis of antibody-coated red cell membranes in the spleen, resulting in spherocyte formation.

Evaluate the 28-year-old Mediterranean man with asymptomatic microcytosis and normal iron studies.

Beta-thalassemia minor is diagnosed.

The altered hemoglobin-to-membrane ratio in thalassemia characteristically produces target cells on peripheral smear.

Determine the underlying condition in the 70-year-old man with massive splenomegaly and a fibrotic marrow (dry tap).

Primary myelofibrosis is diagnosed.

Extramedullary hematopoiesis and marrow fibrosis mechanically distort exiting red blood cells, consistently forming teardrop cells.

Key Concept

Characteristic peripheral blood smear findings directly reflect the pathophysiologic mechanisms of varying anemias, such as oxidative damage (bite cells), membrane loss (spherocytes), altered hemoglobin-to-membrane ratio (target cells), or physical marrow distortion (teardrop cells).

Question 1588Question →

A 60-year-old man is brought to the emergency department by a neighbor who found him lethargic and confused in his apartment. The neighbor notes that the patient has a history of severe alcohol use disorder and has been drinking heavily for the past week. He has also had multiple episodes of non-bilious vomiting over the last three days. Physical examination reveals dry mucous membranes and decreased skin turgor. Laboratory studies are obtained:

Laboratory Study	Result
Serum sodium	$140\text{ mEq/L}$
Serum potassium	$3.2\text{ mEq/L}$
Serum chloride	$88\text{ mEq/L}$
Serum bicarbonate	$16\text{ mEq/L}$
Blood urea nitrogen (BUN)	$45\text{ mg/dL}$
Serum creatinine	$1.8\text{ mg/dL}$
Blood glucose	$65\text{ mg/dL}$

Arterial blood gas on room air reveals a pH of $7.30$ and PaCO $_2$ of $32\text{ mm Hg}$ .

Calculate the patient's delta-delta ratio, defined as the change in anion gap divided by the change in serum bicarbonate ( $\frac{\Delta \text{AG}}{\Delta \text{HCO}_3^-}$ ). Assume a normal anion gap of $12\text{ mEq/L}$ (calculated without potassium) and a normal serum bicarbonate of $24\text{ mEq/L}$ .

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Answer: 3

Answer

The patient presents with alcoholic ketoacidosis (AKA) from heavy alcohol use, which generates a high anion gap metabolic acidosis. He also has a concurrent metabolic alkalosis from severe vomiting (loss of gastric HCl). The delta-delta ratio quantifies the relationship between the accumulation of unmeasured anions and the consumption of bicarbonate. An anion gap of 36 mEq/L yields a ΔAG of 24, while the bicarbonate of 16 mEq/L yields a ΔHCO3 of 8. The resulting ratio of 3.0 (which is > 2.0) mathematically confirms the presence of a superimposed metabolic alkalosis, as the rise in unmeasured anions significantly exceeds the drop in bicarbonate.

Step-by-Step Solution

Calculate the patient's current anion gap (AG).

AG = 140 - (88 + 16) = 36 mEq/L

The anion gap is needed to determine the presence of a high anion gap metabolic acidosis (HAGMA) and to calculate the numerator of the delta-delta ratio.

Calculate the change in anion gap (ΔAG).

ΔAG = Measured AG - Normal AG = 36 - 12 = 24 mEq/L

This value represents the excess unmeasured anions contributing to the acidosis.

Calculate the change in serum bicarbonate (ΔHCO3-).

ΔHCO3- = Normal HCO3- - Measured HCO3- = 24 - 16 = 8 mEq/L

This value represents the amount of bicarbonate buffered by the excess acid.

Calculate the delta-delta ratio.

Delta Ratio = ΔAG / ΔHCO3- = 24 / 8 = 3

The delta-delta ratio helps identify concurrent metabolic acid-base disorders.

Key Concept

The delta-delta ratio is used in the evaluation of high anion gap metabolic acidosis to identify concurrent mixed metabolic acid-base disorders; a ratio greater than 2.0 indicates a superimposed metabolic alkalosis.

Question 1589Question →

During a late June clinic visit in Massachusetts, a $28$ -year-old pregnant woman at $18$ weeks gestation presents for evaluation of an expanding rash. She reports feeling unusually fatigued over the past $4$ days, accompanied by a mild headache. Two weeks ago, the patient spent a weekend camping in a heavily wooded area, though she does not recall any specific insect bites. Her medical history is notable for a childhood episode of anaphylaxis following penicillin administration, which required emergent intubation. Vital signs include a temperature of $37.4^\circ\text{C}$ , heart rate of $88\text{/min}$ , and blood pressure of $110/70\text{ mm Hg}$ . Physical examination reveals a $9\text{-cm}$ annular, erythematous plaque with central clearing on the right posterior thigh. The lesion is warm but non-tender. The remainder of the examination, including a detailed neurological assessment, is unremarkable. Which of the following is the most appropriate pharmacotherapy for this patient?

Oral azithromycin

Oral doxycycline

Oral cefuroxime axetil

Intravenous ceftriaxone

Oral amoxicillin-clavulanate

Show answer & explanation

Answer: Oral azithromycin

Answer

Oral azithromycin

The correct option is oral azithromycin. This patient's presentation of an expanding annular, erythematous plaque with central clearing (erythema migrans) in an endemic area during summer is highly diagnostic for early localized Lyme disease, caused by the spirochete Borrelia burgdorferi. The standard first-line treatments for early localized Lyme disease include oral doxycycline, amoxicillin, or cefuroxime. However, this patient's pregnancy strictly contraindicates the use of doxycycline due to the risk of fetal bone growth inhibition and deciduous tooth discoloration. Furthermore, her history of an IgE-mediated severe allergic reaction to penicillin (anaphylaxis requiring intubation) contraindicates amoxicillin. Cefuroxime, a second-generation cephalosporin, must also be avoided in patients with a history of severe penicillin anaphylaxis due to the significant risk of cross-reactivity. In pregnant patients who cannot safely tolerate amoxicillin or cefuroxime, macrolides such as oral azithromycin are the preferred alternative therapy.

Step-by-Step Solution

Diagnose the patient's condition based on the clinical presentation and epidemiological risk factors.

The patient has early localized Lyme disease, indicated by the classic erythema migrans rash (annular plaque with central clearing) following outdoor exposure in an endemic area (Massachusetts) during summer.

Accurate identification of the infection dictates the specific pathogen targeted (Borrelia burgdorferi).

Evaluate the standard first-line therapies against the patient's specific physiological state (pregnancy).

Standard therapies include doxycycline, amoxicillin, and cefuroxime. Doxycycline is contraindicated due to pregnancy (teratogenic effects on fetal bone and teeth).

Pregnancy significantly alters the safety profile of common antimicrobial agents.

Evaluate the remaining safe options against the patient's medical history (allergies).

The patient has a history of anaphylaxis to penicillin. This completely contraindicates amoxicillin and heavily cautions against cefuroxime (a cephalosporin) due to the risk of severe cross-reactivity. Therefore, a macrolide (azithromycin) becomes the next best step.

Severe IgE-mediated drug reactions necessitate avoiding both the offending drug class and structurally related classes with cross-reactivity potential.

Key Concept

Management of early localized Lyme disease in pregnant patients with severe penicillin allergy

Question 1590Question →

A 58-year-old man is evaluated on hospital day 3 for persistent fever. He was admitted with fever, severe left flank pain, and dysuria, and was diagnosed with acute complicated pyelonephritis. At that time, empiric therapy with intravenous ceftriaxone was initiated.

Today, he reports continued flank pain and malaise. His temperature is $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ), blood pressure is $118/76\text{ mmHg}$ , pulse is $102\text{/min}$ , and respiratory rate is $18\text{/min}$ . Physical examination reveals persistent left costovertebral angle tenderness.

Blood cultures drawn on admission are now positive for *Enterobacter cloacae*. The antimicrobial susceptibility report is as follows:

Antimicrobial Agent	Interpretation
Ampicillin	Resistant
Cefazolin	Resistant
Ceftriaxone	Susceptible
Cefepime	Susceptible
Ciprofloxacin	Susceptible
Meropenem	Susceptible
Piperacillin-tazobactam	Susceptible

Which of the following is the most appropriate next step in pharmacotherapy?

Continue intravenous ceftriaxone

Transition to intravenous meropenem

Add intravenous vancomycin

Transition to intravenous cefepime

Transition to oral ciprofloxacin

Show answer & explanation

Answer: Transition to intravenous cefepime

Answer

Transition to intravenous cefepime is the most appropriate next step to effectively treat this AmpC-producing pathogen while adhering to antimicrobial stewardship.

The correct answer is to transition to intravenous cefepime. This patient has bacteremic pyelonephritis caused by *Enterobacter cloacae*. *Enterobacter* species are part of the 'SPACE' group (Serratia, Pseudomonas, Acinetobacter, Citrobacter, Enterobacter) of Gram-negative bacilli, which possess a chromosomally encoded, inducible AmpC beta-lactamase. When exposed to third-generation cephalosporins (such as ceftriaxone), these bacteria can rapidly upregulate AmpC production. This leads to clinical treatment failure and in vivo resistance, despite the organism appearing susceptible on initial in vitro testing. Therefore, ceftriaxone should never be used for invasive *Enterobacter* infections. According to major infectious disease guidelines, intravenous cefepime (a fourth-generation cephalosporin) is the preferred definitive therapy. Cefepime is highly stable against AmpC hydrolysis, provides excellent targeted coverage, and spares the use of carbapenems, perfectly aligning with the principles of antimicrobial stewardship.

Step-by-Step Solution

Analyze the patient's clinical status and microbiological data.

The patient remains febrile and symptomatic on day 3 of ceftriaxone therapy. Blood cultures have isolated Enterobacter cloacae.

Identifying the clinical trajectory and specific pathogen is required to optimize targeted antimicrobial therapy.

Identify the intrinsic resistance mechanisms of the isolated pathogen.

Enterobacter cloacae is known to possess a chromosomally encoded, inducible AmpC beta-lactamase.

Organisms with inducible AmpC (like the SPACE organisms) can rapidly develop resistance to third-generation cephalosporins in vivo, despite appearing susceptible in vitro.

Select the most appropriate definitive pharmacotherapy.

Cefepime is selected because it is stable against AmpC hydrolysis, avoiding the need for ultra-broad carbapenems.

Choosing cefepime balances therapeutic efficacy with responsible antimicrobial stewardship.

Key Concept

Infections caused by SPACE organisms (e.g., Enterobacter) with inducible AmpC beta-lactamases should be treated with cefepime or carbapenems, avoiding third-generation cephalosporins even if they appear susceptible in vitro.

Estimated Time:1m 30s

Question 1591Question →

A $76$ -year-old woman is brought to the emergency department by her daughter due to worsening confusion and rapid breathing that started earlier today. The daughter notes the patient has severe osteoarthritis and recently started taking a new "over-the-counter powder" for joint pain in addition to her prescribed medications. Temperature is $38.4^\circ\text{C}$ ( $101.1^\circ\text{F}$ ), blood pressure is $110/70 \text{ mm Hg}$ , pulse is $112/\text{min}$ , and respiratory rate is $32/\text{min}$ . Physical examination shows a confused, elderly woman. Mucous membranes are dry. Cardiopulmonary examination reveals clear lungs bilaterally and tachycardia without murmurs. Laboratory studies show:

Leukocyte count: $14,500/\text{mm}^3$
Serum sodium: $140 \text{ mEq/L}$
Serum potassium: $3.8 \text{ mEq/L}$
Serum chloride: $104 \text{ mEq/L}$
Serum bicarbonate: $12 \text{ mEq/L}$
Blood urea nitrogen: $32 \text{ mg/dL}$
Serum creatinine: $1.5 \text{ mg/dL}$
Thyroid-stimulating hormone (TSH): $0.2 \ \mu\text{U/mL}$ (Normal: $0.5-5.0$ )
Free T4: $0.6 \text{ ng/dL}$ (Normal: $0.9-1.7$ )

Arterial blood gas analysis on room air:
pH: $7.50$
$pCO_2$ : $16 \text{ mm Hg}$
$pO_2$ : $98 \text{ mm Hg}$

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

Intravenous administration of sodium bicarbonate

Empiric administration of intravenous ceftriaxone and azithromycin

Rapid intravenous bolus of 0.9% normal saline

Intravenous administration of levothyroxine

Administration of intravenous furosemide

Show answer & explanation

Answer: Intravenous administration of sodium bicarbonate

Answer

Intravenous administration of sodium bicarbonate

The correct answer addresses the specific toxidrome revealed by the patient's arterial blood gas. The patient's acid-base status demonstrates a primary high anion gap metabolic acidosis (AG =

24

) and a primary respiratory alkalosis (actual

pCO_2

16

is lower than the Winter's formula expected range of

24-28

). The alkalemic pH (

7.50

) confirms the respiratory alkalosis is a primary, dominant process rather than merely compensation. This specific mixed acid-base disorder, especially accompanied by fever, confusion, and a history of unprescribed analgesic powder use, is pathognomonic for salicylate (aspirin) toxicity. Salicylates directly stimulate the medullary respiratory center (causing respiratory alkalosis) and uncouple oxidative phosphorylation (leading to lactic acidosis and ketoacidosis, causing HAGMA). Intravenous sodium bicarbonate is the most appropriate next step; it alkalinizes the serum to reduce salicylate entry into the central nervous system and alkalinizes the urine to promote salicylate excretion via ion trapping.

Step-by-Step Solution

Analyze the pH to determine the dominant primary disorder.

The pH is

7.50

(alkalemia), indicating that a primary alkalosis is the dominant driving force.

Compensation for an acid-base disorder never overcorrects the pH past the normal range (

7.35-7.45

Calculate the Anion Gap (AG) to assess for metabolic acidosis.

\text{AG} = Na - (Cl + HCO_3) = 140 - (104 + 12) = 24

A high anion gap (

>12

) confirms the presence of a primary high anion gap metabolic acidosis (HAGMA), regardless of the overall pH.

Apply Winter's formula to determine the expected respiratory compensation for the HAGMA.

Expected

pCO_2 = (1.5 \times HCO_3) + 8 = (1.5 \times 12) + 8 = 26 \pm 2 \text{ mm Hg}

This establishes what the

pCO_2

should be if the HAGMA was the only primary disorder.

Compare actual

pCO_2

to expected

pCO_2

The actual

pCO_2

(

16 \text{ mm Hg}

) is significantly lower than the expected range (

24-28 \text{ mm Hg}

This proves there is a superimposed, independent primary respiratory alkalosis.

Synthesize the clinical presentation with the acid-base diagnosis.

A mixed HAGMA and respiratory alkalosis in an elderly patient with fever, confusion, and use of an unknown analgesic powder is the classic presentation of salicylate (aspirin) toxicity.

Salicylates directly stimulate the medullary respiratory center and uncouple oxidative phosphorylation.

Determine the appropriate management.

Administer intravenous sodium bicarbonate.

Sodium bicarbonate alkalinizes the serum (keeping salicylate out of the CNS) and alkalinizes the urine (promoting salicylate excretion via ion trapping).

Key Concept

Mixed Acid-Base Disorders in Salicylate Toxicity

Alternative Method

When evaluating an ABG, checking the pH first immediately reveals the primary disorder. A pH of 7.50 means alkalosis is the primary driving force. Seeing a low HCO3 (12) means metabolic acidosis is present. A patient cannot have a primary metabolic acidosis driving an alkaline pH. Therefore, the low pCO2 (16) MUST be the primary driver of the alkalemia. The simultaneous presence of an anion gap of 24 proves HAGMA is also present. Mixed HAGMA + Resp Alk = Salicylates.

Estimated Time:2m 0s

Question 1592Question →

A 38-year-old woman is evaluated for progressive proximal muscle weakness, easy bruising, and irregular menses over the past 8 months. Physical examination shows central obesity, a dorsocervical fat pad, and wide purple striae on her abdomen. Blood pressure is 162/98 mm Hg. The physician suspects hypercortisolism and plans a stepwise diagnostic evaluation. Assume the results of each test necessitate further investigation to localize an occult source of adrenocorticotropic hormone (ACTH). Arrange the following diagnostic steps in the correct chronological sequence according to standard endocrine guidelines.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence for evaluating Cushing syndrome is to first confirm hypercortisolism (low-dose dexamethasone suppression test), then determine ACTH dependence (plasma ACTH level), followed by initial localization (pituitary MRI), and finally invasive localization if imaging is inconclusive (inferior petrosal sinus sampling).

The algorithm for diagnosing Cushing syndrome requires a rigid stepwise approach to prevent diagnostic errors caused by physiological ACTH fluctuations and anatomical incidentalomas. The correct sequence is: biochemical confirmation of hypercortisolism, determination of ACTH dependence, non-invasive imaging, and finally invasive biochemical localization.

Step-by-Step Solution

Confirm pathological hypercortisolism

Establishes the biochemical diagnosis of Cushing syndrome

Before localizing a lesion, the diagnosis of hypercortisolism must be biochemically confirmed to avoid false positives from physiological stress or incidentalomas.

Determine ACTH dependence

Differentiates ACTH-dependent (central/ectopic) from ACTH-independent (adrenal) causes

Guides the choice of subsequent imaging. Measuring ACTH too early can be misleading due to pulsatile secretion.

Perform initial source localization

Identifies the anatomical source of ACTH production (e.g., pituitary adenoma)

A pituitary MRI is the first-line imaging modality for ACTH-dependent Cushing syndrome because Cushing disease is the most common etiology.

Perform invasive sampling (if indicated)

Definitively distinguishes pituitary from ectopic ACTH production

Required when MRI is negative or shows a microadenoma smaller than 6 mm, as up to 10% of the population has non-functioning pituitary incidentalomas.

Key Concept

The evaluation of Cushing syndrome follows a strict biochemical and radiographic sequence: 1) Confirm hypercortisolism, 2) Determine ACTH dependence, 3) Image the suspected source, 4) Perform invasive sampling if non-invasive tests are equivocal.

Question 1593Question →

A 38-year-old woman comes to the clinic reporting progressive muscle weakness and fatigue over the past several weeks. She has a past medical history of Sjögren syndrome, for which she uses artificial tears and takes hydroxychloroquine. She denies recent diarrhea, vomiting, or changes in her medications. Vital signs are within normal limits.

Laboratory studies show:
Sodium: $139 \text{ mEq/L}$
Potassium: $2.6 \text{ mEq/L}$
Chloride: $113 \text{ mEq/L}$
Bicarbonate: $14 \text{ mEq/L}$

Arterial blood gas on room air:
pH: $7.26$
PaCO2: $29 \text{ mm Hg}$

Urinalysis reveals a pH of $6.8$ with no glucose, protein, or blood.

Which of the following is the most likely primary mechanism for this patient's acid-base and electrolyte abnormalities?

Defective hydrogen ion secretion in the distal tubule

Decreased proximal tubular reabsorption of bicarbonate

Reduced collecting duct responsiveness to aldosterone

Excessive gastrointestinal loss of bicarbonate-rich fluids

Concomitant primary respiratory alkalosis

Show answer & explanation

Answer: Defective hydrogen ion secretion in the distal tubule

Answer

Defective hydrogen ion secretion in the distal tubule

The correct answer describes the pathophysiology of Type 1 (distal) renal tubular acidosis (RTA). This patient presents with a normal anion gap (

12 \text{ mEq/L}

) metabolic acidosis. In the setting of a normal gap metabolic acidosis, the serum potassium and urine pH are critical for differentiating the etiology. The patient has hypokalemia, which rules out Type 4 RTA (characterized by hyperkalemia). To distinguish between gastrointestinal bicarbonate loss (diarrhea), Type 2 (proximal) RTA, and Type 1 (distal) RTA, the urine pH is evaluated. In a systemic acidemic state, a normal functioning distal nephron will maximize hydrogen ion secretion, lowering the urine pH below 5.5. The fact that this patient's urine pH is 6.8 indicates an inability of the distal tubule to secrete hydrogen ions. Furthermore, Type 1 RTA is strongly associated with autoimmune disorders, most notably Sjögren syndrome and rheumatoid arthritis.

Step-by-Step Solution

Calculate the serum anion gap to determine the type of metabolic acidosis.

Anion Gap =

Na^+ - (Cl^- + HCO_3^-) = 139 - (113 + 14) = 12 \text{ mEq/L}

. This is a normal anion gap (non-anion gap) metabolic acidosis.

Classifying the acidosis narrows the differential diagnosis between anion gap causes (e.g., ketoacidosis, lactic acidosis) and normal gap causes (e.g., renal tubular acidosis, diarrhea).

Calculate the expected respiratory compensation using Winter's formula.

PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5(14) + 8 = 21 + 8 = 29 \pm 2 \text{ mm Hg}

. The measured

PaCO_2

29 \text{ mm Hg}

, indicating appropriate compensation without a mixed disorder.

Verifying respiratory compensation rules out concurrent primary respiratory acidosis or alkalosis.

Evaluate the serum potassium and urine pH.

The patient has severe hypokalemia (

2.6 \text{ mEq/L}

) and an inappropriately basic urine pH (

6.8

In normal gap metabolic acidosis, potassium distinguishes Type 4 RTA (high) from Types 1 and 2 RTA and diarrhea (low/normal). Urine pH distinguishes Type 1 RTA (> 5.5) from Type 2 RTA and diarrhea (< 5.5).

Synthesize the laboratory findings with the clinical presentation.

Normal anion gap metabolic acidosis + hypokalemia + urine pH > 5.5 + Sjögren syndrome indicates Type 1 (distal) renal tubular acidosis, caused by defective hydrogen ion secretion.

Autoimmune conditions like Sjögren syndrome are classic secondary causes of Type 1 RTA due to autoantibody-mediated damage to the distal intercalated cells.

Key Concept

Diagnosis of Type 1 (distal) Renal Tubular Acidosis

Question 1594Question →

A 48-year-old male with a 20-pack-year smoking history is admitted to the hospital with a three-week history of worsening cough, blood-tinged sputum, and profound fatigue. Over the last six months, he has experienced chronic nasal crusting, recurrent epistaxis, and sinus pressure that failed to improve with over-the-counter decongestants. His temperature is 37.9°C (100.2°F), blood pressure is 158/92 mmHg, and pulse is 98/min. Physical examination reveals a mild saddle nose deformity and bilateral lung crackles. Recent outpatient laboratory testing showed a normal thyroid-stimulating hormone (TSH) level of 2.1 µU/mL and a low free thyroxine (T4) level. Current admission laboratory results are as follows:

Test	Result
Blood urea nitrogen	42 mg/dL
Serum creatinine	2.6 mg/dL (baseline 0.9 mg/dL)
Serum bicarbonate	16 mEq/L
Arterial pCO2	35 mmHg

Urinalysis demonstrates 3+ blood, 2+ protein, and numerous dysmorphic red blood cells. A chest radiograph reveals multiple bilateral nodular opacities, several of which have central cavitation. A rapid sputum gram stain from the emergency department is negative for organisms. Which of the following is the most appropriate next step in the patient's care?

Measurement of serum antineutrophil cytoplasmic antibodies

Initiation of empiric broad-spectrum antimicrobial therapy

Administration of intravenous furosemide for acute decompensated heart failure

Initiation of levothyroxine therapy for primary hypothyroidism

Administration of intravenous sodium bicarbonate to correct uncompensated metabolic acidosis

Show answer & explanation

Answer: Measurement of serum antineutrophil cytoplasmic antibodies

Answer

Measurement of serum antineutrophil cytoplasmic antibodies

The patient's presentation is highly characteristic of Granulomatosis with Polyangiitis (GPA, formerly Wegener's granulomatosis). GPA is a small-vessel systemic vasculitis classically presenting with a triad of upper respiratory tract manifestations (recurrent epistaxis, nasal septal perforation resulting in a saddle nose deformity), lower respiratory tract manifestations (hemoptysis, bilateral cavitary lung nodules), and renal manifestations (rapidly progressive glomerulonephritis presenting with acute kidney injury, proteinuria, and dysmorphic red blood cells). The most appropriate next step in the evaluation of a patient with suspected GPA is the measurement of serum antineutrophil cytoplasmic antibodies (ANCA). Approximately 90% of patients with active, generalized GPA will have a positive c-ANCA (directed against proteinase-3, PR3). While a tissue biopsy (often of the lung or kidney) is ultimately required for definitive diagnosis, ANCA testing is the standard initial serologic workup.

Step-by-Step Solution

Analyze the clinical triad presented in the vignette.

The patient has upper respiratory involvement (nasal crusting, saddle nose), lower respiratory involvement (cavitary lung nodules, hemoptysis), and renal involvement (rapidly progressive glomerulonephritis).

Identifying this specific triad of organ systems is crucial for recognizing Granulomatosis with Polyangiitis (GPA).

Rule out alternative etiologies based on laboratory and imaging data.

Infection is less likely given the negative gram stain and chronicity of upper airway symptoms. Heart failure is ruled out by the cavitary nature of the nodules and dysmorphic RBCs in the urine.

Differentiating systemic vasculitis from infectious or cardiopulmonary etiologies prevents inappropriate therapies.

Determine the optimal next step for the suspected autoimmune disease.

Testing for antineutrophil cytoplasmic antibodies (specifically c-ANCA/PR3-ANCA) is the most appropriate initial non-invasive diagnostic test to support the diagnosis of GPA before proceeding to tissue biopsy.

Serologic confirmation guides subsequent aggressive immunosuppressive therapy.

Key Concept

Diagnosis of Granulomatosis with Polyangiitis (GPA)

Question 1595Question →

A 64-year-old male is brought to the emergency department by his wife due to progressively worsening breathing difficulties over the past ten days. The patient reports a persistent dry cough and states he has been sleeping in a recliner because he feels like he is 'smothering' when lying flat in bed. His medical history is notable for severe, erosive rheumatoid arthritis, hypertension, and moderate chronic obstructive pulmonary disease. Four months prior to this presentation, adalimumab was added to his long-standing regimen of methotrexate and naproxen to achieve better control of his joint disease. He also takes amlodipine and uses a tiotropium inhaler daily.

Temperature is 37.0°C (98.6°F), blood pressure is 156/92 mm Hg, pulse is 108/min, and respirations are 24/min. Pulse oximetry indicates an oxygen saturation of 92% on ambient air. Physical examination reveals jugular venous distension up to 8 cm above the sternal angle. Auscultation of the chest demonstrates a regular tachycardia with a prominent S3 gallop, alongside fine bilateral basilar crackles. No wheezes are appreciated. The lower extremities exhibit 2+ pitting edema extending to the mid-calves.

Laboratory analysis reveals:
- Hemoglobin: 11.2 g/dL
- Leukocyte count: 8,500/mm³
- Thyroid-stimulating hormone: 1.5 µU/mL
- Free thyroxine (T4): 1.3 ng/dL

Arterial blood gas analysis on ambient air:
- pH: 7.48
- PaCO₂: 30 mm Hg
- PaO₂: 64 mm Hg
- HCO₃⁻: 22 mEq/L

Based on the clinical findings, which of the following is the most appropriate next step in the management of this patient?

Administer systemic glucocorticoids and nebulized albuterol

Obtain blood cultures and initiate intravenous piperacillin-tazobactam

Discontinue adalimumab and administer intravenous furosemide

Administer intravenous acetazolamide to correct the acid-base derangement

Discontinue methotrexate and initiate high-dose corticosteroid therapy

Show answer & explanation

Answer: Discontinue adalimumab and administer intravenous furosemide

Answer

Discontinue adalimumab and administer intravenous furosemide

The correct answer involves discontinuing the TNF-alpha inhibitor (adalimumab) and initiating diuresis (furosemide). This patient presents with classic signs of acute decompensated heart failure (ADHF), evidenced by orthopnea, jugular venous distension, an S3 gallop, bilateral basilar crackles, and peripheral edema. Adalimumab and other TNF-alpha inhibitors (e.g., infliximab, etanercept) are biologic disease-modifying antirheumatic drugs (DMARDs) associated with a significant risk of precipitating new-onset heart failure or exacerbating pre-existing heart failure. They are absolutely contraindicated in patients with New York Heart Association (NYHA) Class III or IV heart failure. Given the clear signs of volume overload and ADHF occurring relatively soon after the initiation of adalimumab, the offending drug must be stopped immediately, and the pulmonary congestion and peripheral edema should be managed with loop diuretics.

### Clinical Differentiation of Dyspnea in Rheumatoid Arthritis

Feature	Acute Heart Failure (TNF-alpha induced)	COPD Exacerbation	Methotrexate Pneumonitis
Key Symptoms	Orthopnea, PND, progressive dyspnea	Dyspnea, increased sputum purulence	Dry cough, progressive dyspnea
Exam Findings	JVD, S3 gallop, edema, crackles	Wheezing, prolonged expiration	Crackles, absence of volume overload
Acid-Base	Respiratory alkalosis (early)	Respiratory acidosis (often)	Respiratory alkalosis
Management	Diuretics, stop biologic agent	Bronchodilators, systemic steroids	Stop MTX, systemic steroids

Step-by-Step Solution

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

The patient exhibits orthopnea, jugular venous distension, an S3 gallop, basilar crackles, and bilateral pitting edema.

Identifying this specific constellation of signs and symptoms is necessary to establish that the primary clinical syndrome is acute decompensated heart failure (ADHF) with volume overload.

Review the patient's medical history and recent medication changes in the context of the new diagnosis.

The patient was started on adalimumab, a tumor necrosis factor (TNF)-alpha inhibitor, four months prior for severe rheumatoid arthritis.

TNF-alpha inhibitors are a well-documented cause of new-onset or worsening heart failure, making this drug the most likely precipitant of his current ADHF.

Determine the most appropriate immediate management for both the acute presentation and the underlying pharmacological cause.

The offending agent (adalimumab) must be discontinued, and the acute volume overload must be treated with a loop diuretic (intravenous furosemide).

Removing the pharmacological trigger and actively diuresing the patient are the fundamental cornerstones of managing drug-induced heart failure exacerbations.

Key Concept

Tumor necrosis factor (TNF)-alpha inhibitors (e.g., adalimumab, infliximab, etanercept) can precipitate new-onset heart failure or exacerbate pre-existing disease. They are contraindicated in patients with moderate-to-severe heart failure.

Question 1596Question →

A 38-year-old mechanic is evaluated in the hospital after being found confused and lethargic in his workshop. He has a history of alcohol use disorder. Vital signs show a temperature of 37.1°C (98.8°F), blood pressure of 135/85 mmHg, pulse of 92/min, and respiratory rate of 22/min. Physical examination reveals a disoriented patient with dry mucous membranes.

Initial laboratory evaluation reveals the following:

Parameter	Value
Sodium	140 mEq/L
Potassium	4.1 mEq/L
Chloride	98 mEq/L
Bicarbonate	14 mEq/L
Blood urea nitrogen	20 mg/dL
Serum creatinine	1.5 mg/dL
Glucose	105 mg/dL

Based on the provided laboratory results, what is this patient's serum anion gap?

Show answer & explanation

Answer: 28

Answer

The serum anion gap is calculated by subtracting the primary measured serum anions (chloride and bicarbonate) from the primary measured serum cation (sodium). Using the formula Anion Gap =

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

, the calculation for this patient is

140 - (98 + 14) = 140 - 112 = 28

mEq/L. A normal anion gap is typically between 8 and 12 mEq/L. This patient has a significantly elevated high anion gap metabolic acidosis (HAGMA), which is consistent with toxic alcohol ingestion (such as methanol or ethylene glycol), given his presentation and history.

Step-by-Step Solution

Identify the formula for the serum anion gap.

Anion Gap =

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

The standard serum anion gap is calculated using the primary measured serum cation and the primary measured serum anions.

Extract the required laboratory values from the clinical vignette.

Na^+

= 140 mEq/L,

Cl^-

= 98 mEq/L, and

HCO_3^-

= 14 mEq/L.

These specific values are required to populate the formula.

Calculate the sum of the measured anions.

98 + 14 = 112

mEq/L

This determines the total measured negative charge to be subtracted from sodium.

Subtract the sum of the anions from the serum sodium concentration.

140 - 112 = 28

mEq/L

This yields the final unmeasured anion gap.

Key Concept

Calculation of the serum anion gap to evaluate metabolic acidosis.

Question 1597Question →

Match the following laboratory profiles regarding serum calcium and intact parathyroid hormone (PTH) to their most likely underlying diagnosis.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Elevated calcium and elevated PTH matches with Primary hyperparathyroidism. Elevated calcium and suppressed PTH matches with Hypercalcemia of malignancy. Decreased calcium and elevated PTH matches with Secondary hyperparathyroidism. Decreased calcium and low PTH matches with Primary hypoparathyroidism.

Matching laboratory profiles of calcium and PTH directly tests the understanding of parathyroid gland function and its feedback loop. When the parathyroid autonomously produces excess PTH, both calcium and PTH are high (primary hyperparathyroidism). When calcium is high due to other causes like malignancy, normal feedback suppresses PTH. When calcium is low, a healthy parathyroid gland increases PTH production (secondary hyperparathyroidism). If the gland is destroyed or removed, both calcium and PTH will be low (hypoparathyroidism).

Step-by-Step Solution

Analyze the relationship between serum calcium and intact parathyroid hormone (PTH) in each laboratory profile.

Identify whether the PTH level is an appropriate physiological response to the calcium level or if it is the primary driver of the abnormality.

Calcium and PTH operate on a tight negative feedback loop; understanding this relationship is the key to diagnosing the underlying endocrine disorder.

Evaluate the profiles presenting with hypercalcemia.

High calcium paired with high PTH indicates primary hyperparathyroidism. High calcium paired with suppressed PTH indicates a non-parathyroid cause, most classically hypercalcemia of malignancy.

Autonomous PTH secretion drives calcium up in primary hyperparathyroidism, whereas in malignancy, the high calcium successfully suppresses healthy parathyroid tissue.

Evaluate the profiles presenting with hypocalcemia.

Low calcium paired with high PTH indicates a compensatory response known as secondary hyperparathyroidism. Low calcium paired with low PTH indicates failure of the parathyroid glands (primary hypoparathyroidism).

A healthy parathyroid gland will predictably respond to hypocalcemia by increasing PTH secretion. The absence of this response confirms gland failure.

Key Concept

The relationship between serum calcium and intact parathyroid hormone is determined by negative feedback; deviations from this relationship differentiate primary, secondary, and non-parathyroid disorders of calcium metabolism.

Question 1598Question →

Which primary pathophysiologic mechanism corresponds to each of the following patient presentations occurring during or shortly after a blood product transfusion?

Click a left item, then click its matching right item

Show answer & explanation

Answer

IgA deficiency causes anaphylaxis (anti-IgA); ABO incompatibility causes acute hemolysis (anti-donor RBC); volume overload causes TACO (hydrostatic pressure); donor antibodies cause TRALI (neutrophil activation); and storage cytokines cause febrile non-hemolytic reactions.

Each acute transfusion reaction has a distinct pathophysiologic trigger. Anaphylaxis occurs in IgA-deficient individuals who have formed anti-IgA. Acute hemolytic reactions are due to ABO mismatch causing rapid intravascular hemolysis. TACO is simply hydrostatic volume overload in susceptible individuals. TRALI is immune-mediated noncardiogenic pulmonary edema driven by donor anti-leukocyte antibodies. Febrile non-hemolytic reactions are caused by cytokines accumulated during storage.

Step-by-Step Solution

Analyze the timing and clinical signs of the patient with recurrent infections.

Rapid onset hypotension and wheezing indicates an anaphylactic transfusion reaction.

Patients with IgA deficiency form anti-IgA antibodies that react instantly to donor IgA.

Evaluate the patient featuring flank pain and hemoglobinuria.

Symptoms align with acute intravascular hemolysis.

Preformed recipient antibodies (usually anti-ABO) rapidly destroy donor erythrocytes, releasing hemoglobin.

Compare the two patients presenting with respiratory distress post-transfusion.

The patient with low ejection fraction and jugular venous distention has cardiogenic edema (TACO), while the patient with normal central venous pressure has noncardiogenic edema (TRALI).

Distinguishing hydrostatic volume overload (TACO) from immune-mediated neutrophil activation (TRALI) is critical in assessing pulmonary complications.

Assess the patient with an isolated mild fever.

Matches the benign profile of a febrile non-hemolytic transfusion reaction.

Confirms the role of accumulated storage cytokines rather than an acute hemolytic process.

Key Concept

Pathophysiology and clinical differentiation of acute blood transfusion reactions

Question 1599Question →

A 54-year-old man is brought to the emergency department by his wife due to a severe, generalized headache, subjective fever, and progressive confusion over the past 24 hours. He has a history of a deceased-donor renal transplant 2 years ago and currently takes tacrolimus and mycophenolate mofetil. His temperature is 39.2°C (102.6°F), blood pressure is 110/70 mmHg, and heart rate is 112/min. On physical examination, the patient is lethargic and oriented only to person. He has marked nuchal rigidity. No papilledema is appreciated, but a detailed neurologic examination is limited by his altered mental status. Arrange the following steps in the most appropriate chronological sequence for the optimal initial management of this patient.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence is to first obtain blood cultures, immediately administer empiric antibiotics and dexamethasone, then perform a noncontrast head CT, and finally perform a lumbar puncture.

In patients with suspected bacterial meningitis, empiric antibiotics should ideally be given after a lumbar puncture. However, if the patient has 'red flag' criteria—such as altered mental status, immunocompromised state, focal neurologic deficits, history of CNS disease, or new-onset seizures—a head CT must be performed first to rule out mass effect and prevent herniation. In this scenario, antibiotics (and dexamethasone) must be administered immediately after drawing blood cultures and BEFORE sending the patient to the CT scanner. Delaying antibiotics for neuroimaging is a fatal error. Therefore, the sequence is: blood cultures, empiric therapy, CT scan, and finally lumbar puncture.

Step-by-Step Solution

Identify the patient's high-risk features for bacterial meningitis.

The patient has signs of meningitis (fever, headache, nuchal rigidity) alongside altered mental status and an immunocompromised state (renal transplant).

Recognizing these features dictates that neuroimaging is required prior to performing a lumbar puncture to prevent cerebral herniation.

Prioritize initial diagnostics that do not delay treatment.

Obtain blood cultures first.

Blood cultures must be drawn before giving antibiotics to increase the chance of isolating the offending pathogen, which is achievable within minutes and does not delay treatment.

Administer life-saving pharmacotherapy immediately.

Give empiric intravenous antibiotics and dexamethasone.

Mortality in bacterial meningitis increases significantly if antibiotics are delayed. Therapy must never be held while waiting for a CT scan or a lumbar puncture.

Sequence the remaining diagnostic procedures safely.

Perform the head CT, followed by the lumbar puncture.

The CT scan must rule out significant mass effect before the lumbar puncture can be safely performed to analyze the cerebrospinal fluid.

Key Concept

Management algorithm for suspected bacterial meningitis in patients with indications for cranial imaging prior to lumbar puncture.

Question 1600Question →

A 62-year-old man is brought to the emergency department by his wife due to worsening lethargy and confusion over the past 24 hours. The patient has a history of alcohol-associated cirrhosis, portal hypertension, and ascites. His wife reports that he has had 6 to 8 large-volume, watery bowel movements daily for the past four days after taking a new over-the-counter laxative. He has also missed his last several doses of lactulose. Vital signs are temperature 36.8 C (98.2 F), blood pressure 95/60 mm Hg, pulse 102/min, and respirations 12/min. Physical examination reveals a somnolent patient who arouses to a loud voice but is oriented only to person. Scleral icterus is present. The abdomen is markedly distended, tense, and dull to percussion with a positive fluid wave. Asterixis is elicited on neurologic examination. Lung auscultation reveals decreased breath sounds at the bases bilaterally, without crackles or wheezes.

Laboratory results:
Sodium: 136 mEq/L
Potassium: 3.2 mEq/L
Chloride: 113 mEq/L
Bicarbonate: 15 mEq/L
Blood urea nitrogen: 35 mg/dL
Creatinine: 1.4 mg/dL

Arterial blood gas (room air):
pH: 7.14
$pCO_2$ : 45 mm Hg
$pO_2$ : 68 mm Hg

Which of the following best describes this patient's acid-base status?

Normal anion gap metabolic acidosis and concurrent respiratory acidosis

Normal anion gap metabolic acidosis with appropriate respiratory compensation

High anion gap metabolic acidosis and concurrent respiratory acidosis

Primary respiratory acidosis with appropriate metabolic compensation

Normal anion gap metabolic acidosis and concurrent respiratory alkalosis

Show answer & explanation

Answer: Normal anion gap metabolic acidosis and concurrent respiratory acidosis

Answer

Normal anion gap metabolic acidosis and concurrent respiratory acidosis

This patient has a normal anion gap (non-anion gap) metabolic acidosis and a concurrent respiratory acidosis. The primary metabolic acidosis is indicated by severe acidemia (pH 7.14) and low bicarbonate (15 mEq/L), driven by severe gastrointestinal bicarbonate loss from laxative-induced diarrhea. The calculated anion gap is 8 mEq/L (

136 - [113 + 15]

), which is within the normal range of 8 to 12 mEq/L. To evaluate for a mixed disorder, Winter's formula (

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

) is used. For this patient, the expected

pCO_2

30.5 \pm 2

mm Hg. The patient's actual measured

pCO_2

is 45 mm Hg. Because the measured

pCO_2

is substantially greater than the expected range, a concurrent respiratory acidosis is present. Clinically, this hypoventilation is explained by tense ascites mechanically restricting diaphragmatic excursion and hepatic encephalopathy depressing his central respiratory drive.

Step-by-Step Solution

Evaluate the primary acid-base disturbance based on pH,

pCO_2

, and

HCO_3^-

The pH is 7.14 (acidemia),

HCO_3^-

is 15 mEq/L (low), and

pCO_2

is 45 mm Hg (high).

Identifying the direction of pH change and correlating it with

HCO_3^-

and

pCO_2

determines the primary disorder. Since both parameters are altered in acidotic directions, this is a mixed disorder.

Calculate the anion gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 136 - (113 + 15) = 8 \text{ mEq/L}

Determining the AG categorizes the metabolic acidosis into high or normal (non-anion gap), narrowing the differential diagnosis. A normal AG is typically 8-12 mEq/L.

Apply Winter's formula to assess respiratory compensation.

Expected\ pCO_2 = 1.5 \times 15 + 8 \pm 2 = 30.5 \pm 2 \text{ mm Hg}

. The measured

pCO_2

is 45 mm Hg, which is significantly higher than expected.

Winter's formula determines if the respiratory response to metabolic acidosis is adequate. A measured

pCO_2

higher than the expected range indicates a concurrent respiratory acidosis.

Key Concept

Acid-base disorders with mixed primary etiologies require systematic calculation of the anion gap and expected respiratory compensation using Winter's formula.

Estimated Time:1m 30s

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Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Answer

Step-by-Step Solution