Internal Medicine Practice Questions — USMLE Step 2 CK — Page 88

Match each clinical presentation with its most likely primary acid-base disorder.

Click a left item, then click its matching right item

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Answer

The presentations match as follows: missed insulin doses with High anion gap metabolic acidosis; COPD hypoventilation with Respiratory acidosis; panic attack hyperventilation with Respiratory alkalosis; intractable vomiting with Metabolic alkalosis.

Each clinical scenario describes a classic, straightforward presentation for one of the four primary acid-base disorders. Identifying the fundamental underlying mechanism—such as ketoacid overproduction, carbon dioxide retention, carbon dioxide washout, or direct acid loss—is essential for accurate diagnosis.

Step-by-Step Solution

1

Analyze the patient with type 1 diabetes and severe hyperglycemia.

The clinical picture indicates diabetic ketoacidosis.

This condition is characterized by the production of ketoacids, which are unmeasured anions that consume bicarbonate and widen the anion gap.

2

Analyze the patient with a severe COPD exacerbation and hypoventilation.

The decreased respiratory drive leads to carbon dioxide retention.

Retained carbon dioxide acts directly as an acid in the blood, leading to a primary respiratory acidosis.

3

Analyze the patient with acute hyperventilation from a panic attack.

The rapid breathing causes excessive elimination of carbon dioxide.

Loss of carbon dioxide raises the blood pH, resulting in a primary respiratory alkalosis.

4

Analyze the patient with intractable vomiting.

Vomiting leads to the continuous loss of gastric contents.

Gastric fluid is rich in hydrochloric acid. Losing this acid directly increases the serum pH, causing a primary metabolic alkalosis.

Key Concept

Primary Acid-Base Disorders

Question 1742Question →

Match each patient's clinical presentation with the most likely predominant finding on their peripheral blood smear.

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Answer

The correct matches are: The 38-year-old female (TTP) pairs with Schistocytes; the 62-year-old male (Hairy Cell Leukemia) pairs with Mononuclear cells with fine projections; the 28-year-old male (G6PD deficiency) pairs with Heinz bodies and bite cells; and the 68-year-old male (Multiple Myeloma) pairs with Rouleaux formation.

The correct pairings successfully match distinct clinical syndromes with their pathognomonic peripheral blood smear findings: TTP causes microangiopathic hemolysis (schistocytes), Hairy cell leukemia presents with pancytopenia and 'hairy' mononuclear cells, G6PD deficiency leads to oxidative damage (bite cells and Heinz bodies), and Multiple Myeloma causes excess serum proteins that alter RBC charge (Rouleaux formation).

Step-by-Step Solution

1

Analyze the first clinical vignette regarding the 38-year-old female.

Identify the constellation of fever, neurologic changes, acute kidney injury, thrombocytopenia, and hemolytic anemia with normal coagulation as Thrombotic Thrombocytopenic Purpura (TTP).

Recognizing the TTP pentad is required to determine the expected peripheral smear finding, which is mechanically sheared red blood cells (schistocytes).

2

Evaluate the presentation of the 62-year-old male with splenomegaly.

Correlate the massive splenomegaly, pancytopenia, and 'dry tap' with Hairy Cell Leukemia.

These triad of findings are hallmark features of Hairy Cell Leukemia, directing the match to cells with fine, thread-like projections.

3

Assess the 28-year-old male's reaction to trimethoprim-sulfamethoxazole.

Diagnose an acute hemolytic crisis due to G6PD deficiency, triggered by oxidative stress.

Oxidative damage in G6PD deficiency leads to precipitated hemoglobin (Heinz bodies) and subsequent macrophage removal of these inclusions (bite cells).

4

Examine the lab values and symptoms of the 68-year-old male.

Identify the CRAB criteria (hypercalcemia, renal failure, anemia, bone pain) and protein gap as indicative of Multiple Myeloma.

High levels of monoclonal proteins in Multiple Myeloma alter red blood cell surface charges, leading to their aggregation into linear stacks (Rouleaux formation).

Key Concept

Characteristic peripheral blood smear findings in hematologic and oncologic disorders

Question 1743Question →

Match each clinical presentation and disease association to the most likely corresponding nephrotic syndrome etiology.

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Answer

Solid tumors correlate with Membranous nephropathy, HIV infection with FSGS, Hepatitis C with MPGN, and childhood post-viral onset with Minimal change disease.

Each type of nephrotic syndrome has classic epidemiological and systemic associations. Identifying these triggers (e.g., solid tumors, specific viral infections, or age demographics) is critical for determining the specific glomerular pathology and guiding subsequent management.

Step-by-Step Solution

1

Analyze the adult presentation with a solid tumor and thickened capillary loops.

This profile maps to Membranous nephropathy.

Membranous nephropathy is the classic nephrotic syndrome paraneoplastic manifestation of solid organ malignancies.

2

Analyze the patient with HIV and focal sclerotic lesions.

This profile maps to Focal segmental glomerulosclerosis (FSGS).

HIV-associated nephropathy (HIVAN) is a highly specific, rapidly progressive form of collapsing FSGS.

3

Analyze the Hepatitis C patient with tram-track glomerular splitting.

This profile maps to Membranoproliferative glomerulonephritis (MPGN).

Hepatitis C drives immune-complex deposition in the glomeruli, characteristic of type I MPGN.

4

Analyze the pediatric patient with post-viral edema.

This profile maps to Minimal change disease.

The sudden onset of pure nephrotic syndrome in a child after a viral illness is the hallmark of Minimal change disease.

Key Concept

Secondary causes and systemic associations of nephrotic syndromes

Question 1744Question →

A physician is evaluating the recent diagnostic workup of a male patient in his early fifties presenting with treatment-resistant hypertension and persistent hypokalemia. The patient is currently prescribed maximum tolerated doses of amlodipine, valsartan, and chlorthalidone. His blood pressure in the clinic is $164/98$ mm Hg.

Initial laboratory evaluation revealed a serum potassium level of $2.9$ mEq/L. The plasma aldosterone concentration (PAC) was $32$ ng/dL, and the plasma renin activity (PRA) was $0.1$ ng/mL/hr. Following an intravenous infusion of $2$ liters of normal saline over $4$ hours, the PAC remained elevated at $19$ ng/dL (normal $< 5$ ng/dL).

Subsequent cross-sectional imaging of the abdomen with a dedicated adrenal protocol CT scan demonstrated a $1.6$ -cm, lipid-rich adenoma in the left adrenal gland. The right adrenal gland appeared morphologically normal.

To further guide management, adrenal venous sampling (AVS) was performed under cosyntropin stimulation. The results are as follows:

Source	Cortisol ( $\mu$ g/dL)	Aldosterone (ng/dL)
Inferior vena cava	$18$	$18$
Right adrenal vein	$360$	$720$
Left adrenal vein	$450$	$1350$

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

Initiation of spironolactone therapy

Left laparoscopic adrenalectomy

Bilateral laparoscopic adrenalectomy

Repeat adrenal venous sampling

Initiation of dexamethasone therapy

Show answer & explanation

Answer: Initiation of spironolactone therapy

Answer

Initiation of spironolactone therapy

The correct answer is the initiation of spironolactone therapy. This patient's presentation of treatment-resistant hypertension and hypokalemia, coupled with an elevated plasma aldosterone concentration (PAC), suppressed plasma renin activity (PRA), and failure of PAC to suppress following intravenous saline loading, confirms the diagnosis of primary aldosteronism.

In patients over

40

years of age with confirmed primary aldosteronism, cross-sectional imaging (CT or MRI) is insufficient to direct surgical therapy because non-functioning adrenal incidentalomas are highly prevalent. Therefore, Adrenal Venous Sampling (AVS) is the gold standard required to differentiate a unilateral aldosterone-producing adenoma from bilateral idiopathic adrenal hyperplasia.

Interpreting AVS requires a systematic approach:
1. Confirm cannulation: The cortisol step-up from the IVC to the adrenal veins must be

>4

(or

>5

with cosyntropin). Here, the right gradient is

360/18 = 20

and the left is

450/18 = 25

, confirming proper cannulation.
2. Correct for dilution: Calculate the aldosterone-to-cortisol ratio for each side. Right =

720/360 = 2

. Left =

1350/450 = 3

.
3. Determine lateralization: Calculate the Lateralization Index (LI) by dividing the dominant side ratio by the non-dominant side ratio.

LI = 3 / 2 = 1.5

.

An LI

> 4

confirms a unilateral source, whereas an LI

< 3

indicates bilateral hypersecretion. Because this patient's LI is

1.5

, he has bilateral idiopathic adrenal hyperplasia. The

1.6

-cm nodule on the left is a non-functioning incidentaloma. The optimal management for bilateral disease is medical therapy with a mineralocorticoid receptor antagonist, such as spironolactone or eplerenone.

Step-by-Step Solution

1

Verify successful cannulation of both adrenal veins during Adrenal Venous Sampling (AVS).

The cortisol gradient between the right adrenal vein and IVC is

360 / 18 = 20

. The gradient between the left adrenal vein and IVC is

450 / 18 = 25

.

A gradient greater than

4

(or

5

under cosyntropin stimulation) confirms that the adrenal veins were successfully cannulated and the samples are valid.

2

Calculate the aldosterone-to-cortisol ratio for each adrenal vein.

Right side ratio:

720 / 360 = 2

. Left side ratio:

1350 / 450 = 3

.

Cortisol levels are used to correct for the dilution of adrenal venous blood by peripheral blood from the inferior vena cava.

3

Calculate the Lateralization Index (LI) to determine the source of aldosterone hypersecretion.

The LI is the ratio of the dominant side to the non-dominant side:

3 / 2 = 1.5

.

An LI

> 4

indicates a unilateral aldosterone-producing adenoma, while an LI

< 3

indicates bilateral idiopathic adrenal hyperplasia. An LI of

1.5

confirms bilateral disease.

4

Select the appropriate management for bilateral idiopathic adrenal hyperplasia.

Initiation of a mineralocorticoid receptor antagonist, such as spironolactone or eplerenone.

Bilateral adrenalectomy is contraindicated due to obligate adrenal insufficiency. Medical management is the definitive standard of care for bilateral disease.

Key Concept

Interpretation of Adrenal Venous Sampling (AVS) in primary aldosteronism to distinguish between a unilateral adenoma and bilateral hyperplasia.

Question 1745Question →

A 54-year-old man with a history of severe chronic obstructive pulmonary disease and major depressive disorder is brought to the emergency department by paramedics after being found unarousable in his garage. An open, empty container of antifreeze was found nearby. Vital signs are temperature 36.7°C (98.1°F), blood pressure 112/68 mmHg, pulse 110/min, and respirations 16/min. Pulse oximetry is 89% on room air. Physical examination reveals an obtunded patient with a barrel-shaped chest, diminished breath sounds, and diffuse bilateral wheezing.

Laboratory testing yields the following results:

Laboratory Test	Value
Sodium	142 mEq/L
Potassium	4.8 mEq/L
Chloride	106 mEq/L
Bicarbonate	12 mEq/L
Arterial pH	7.21
Arterial $pCO_2$	36 mmHg
Arterial $pO_2$	62 mmHg

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

Primary metabolic acidosis with appropriate respiratory compensation

Primary respiratory acidosis with appropriate metabolic compensation

Mixed metabolic acidosis and respiratory acidosis

Mixed metabolic acidosis and respiratory alkalosis

Mixed metabolic acidosis and metabolic alkalosis

Show answer & explanation

Answer: Mixed metabolic acidosis and respiratory acidosis

Answer

Mixed metabolic acidosis and respiratory acidosis

The correct answer accurately identifies two distinct acid-base disorders in this patient. First, the low pH (7.21) and low bicarbonate (12 mEq/L) with an elevated anion gap of 24 mEq/L confirm a high anion gap metabolic acidosis, consistent with toxic alcohol ingestion. Second, applying Winter's formula dictates that his expected

pCO_2

should be between 24 and 28 mmHg. Because his actual

pCO_2

is 36 mmHg, he is retaining more

CO_2

than he should, indicating an independent, concurrent respiratory acidosis caused by his baseline severe COPD.

Step-by-Step Solution

1

Determine the primary acid-base disorder by looking at the pH,

pCO_2

, and

HCO_3^-

.

The pH is 7.21 (acidemia). The

HCO_3^-

is 12 mEq/L (low). This establishes a primary metabolic acidosis.

Acidemia driven by a low bicarbonate indicates a primary metabolic acidosis, which in this clinical context is likely due to ethylene glycol toxicity.

2

Calculate the anion gap to further characterize the metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (106 + 12) = 142 - 118 = 24

mEq/L.

An elevated anion gap (normal 8-12 mEq/L) confirms a high anion gap metabolic acidosis (HAGMA).

3

Calculate the expected respiratory compensation using Winter's formula.

Expected

pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2

mmHg (Range: 24 to 28 mmHg).

Winter's formula determines the appropriate ventilatory response to a metabolic acidosis.

4

Compare the actual

pCO_2

to the expected

pCO_2

to identify any secondary respiratory disorders.

The actual measured

pCO_2

is 36 mmHg. Since 36 mmHg is greater than the upper limit of the expected range (28 mmHg), the patient is hypoventilating relative to their acidemic state.

A

pCO_2

higher than the expected compensatory range indicates a concurrent primary respiratory acidosis, likely due to this patient's severe underlying COPD limiting his ability to appropriately blow off

CO_2

.

Key Concept

In patients with metabolic acidosis, Winter's formula (

1.5 \times [HCO_3^-] + 8 \pm 2

) must be used to calculate expected

pCO_2

. If the measured

pCO_2

is significantly higher than expected, a concurrent respiratory acidosis is present.

Question 1746Question →

A $35$ -year-old man comes to the clinic accompanied by his wife due to severe fatigue, cold intolerance, and weight gain over the past $4$ months. He also reports decreased libido and difficulty sustaining erections. His medical history is significant for a severe traumatic brain injury sustained in a motor vehicle collision $6$ months ago, which resulted in a diffuse axonal injury and a $2$ -week stay in the intensive care unit. His only medication is acetaminophen for occasional headaches. Vital signs show a temperature of $36.1^\circ$ C ( $97.0^\circ$ F), blood pressure of $102/64$ mmHg, and a pulse of $54$ /min. Physical examination reveals dry, pale skin, loss of axillary hair, and delayed relaxation of the deep tendon reflexes.

Laboratory results are as follows:

Test	Result	Reference Range
Serum sodium	$131$ mEq/L	$136$ - $145$ mEq/L
Serum potassium	$4.2$ mEq/L	$3.5$ - $5.0$ mEq/L
Thyroid-stimulating hormone (TSH)	$1.4$ mU/L	$0.4$ - $4.2$ mU/L
Free thyroxine (T4)	$0.5$ ng/dL	$0.9$ - $1.7$ ng/dL

Based on the patient's current condition, which of the following additional laboratory findings is most likely present?

Elevated anti-thyroid peroxidase antibodies

Elevated serum luteinizing hormone (LH)

Decreased serum morning cortisol

Elevated serum adrenocorticotropic hormone (ACTH)

Decreased serum aldosterone

Show answer & explanation

Answer: Decreased serum morning cortisol

Answer

Decreased serum morning cortisol

The correct answer is decreased serum morning cortisol. This patient presents with fatigue, weight gain, cold intolerance, and hypogonadism. His laboratory results show a low Free T4 (

0.5

ng/dL) with an inappropriately normal TSH (

1.4

mU/L), establishing the diagnosis of central (secondary) hypothyroidism. Given his history of a severe traumatic brain injury (TBI), these findings are highly indicative of post-traumatic panhypopituitarism. TBI can cause damage to the pituitary gland or infundibular stalk through shear forces, direct trauma, or vascular compromise. Panhypopituitarism involves the deficiency of multiple anterior pituitary hormones. Along with TSH and gonadotropins (LH/FSH), adrenocorticotropic hormone (ACTH) production is impaired. The lack of ACTH stimulation leads to central adrenal insufficiency, which manifests as a decreased serum morning cortisol level. The patient's euvolemic hyponatremia (

131

mEq/L) with normal potassium is also characteristic of isolated cortisol deficiency, as cortisol normally inhibits antidiuretic hormone (ADH) release.

Step-by-Step Solution

1

Analyze the patient's history and symptoms

The patient has a history of severe traumatic brain injury (TBI) 6 months ago and now presents with symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, delayed reflexes) and hypogonadism (decreased libido, loss of axillary hair).

To identify potential systemic etiologies for his multiple endocrine complaints.

2

Interpret the thyroid function tests

The Free T4 is low (

0.5

ng/dL) with an inappropriately normal TSH (

1.4

mU/L).

To classify the hypothyroidism as primary (thyroid origin) or central/secondary (pituitary/hypothalamic origin).

3

Synthesize findings to establish a unifying diagnosis

The combination of central hypothyroidism, symptoms of hypogonadism, and a severe prior TBI points directly to post-traumatic panhypopituitarism.

TBI is a well-known cause of anterior pituitary dysfunction due to stalk shearing or vascular compromise.

4

Predict other expected laboratory abnormalities

Along with TSH and gonadotropins, ACTH production is likely impaired, which would lead to central adrenal insufficiency and decreased morning cortisol.

Panhypopituitarism typically involves multiple anterior pituitary axes, and recognizing concomitant adrenal insufficiency is critical for safe management.

Key Concept

Post-traumatic panhypopituitarism presenting as central hypothyroidism and central adrenal insufficiency

Question 1747Question →

A 72-year-old woman is admitted to the intensive care unit with acute respiratory distress syndrome secondary to severe community-acquired pneumonia. She is currently intubated and requires vasopressor support for hemodynamic stability. Her medical history is significant for hypertension. There is no personal or family history of endocrine disorders. On hospital day 3, a routine metabolic evaluation includes thyroid function testing with the following results:

Laboratory Parameter	Patient Value	Reference Range
Thyroid-stimulating hormone (TSH)	$1.4 \ \mu\text{U/mL}$	$0.5 - 5.0 \ \mu\text{U/mL}$
Free thyroxine (T4)	$1.2 \ \text{ng/dL}$	$0.9 - 1.7 \ \text{ng/dL}$
Total triiodothyronine (T3)	$45 \ \text{ng/dL}$	$80 - 200 \ \text{ng/dL}$

Which of the following is the most likely explanation for this patient's thyroid function test abnormalities?

Decreased peripheral conversion of prohormone to active hormone

Autoimmune-mediated destruction of thyroid follicular cells

Insufficient pituitary production of thyroid-stimulating hormone

Suppressed pituitary axis from exogenous hormone ingestion

Reduced end-organ responsiveness to circulating thyroid hormone

Show answer & explanation

Answer: Decreased peripheral conversion of prohormone to active hormone

Answer

Decreased peripheral conversion of prohormone to active hormone

The correct answer identifies the mechanism of euthyroid sick syndrome (also known as non-thyroidal illness syndrome). In the setting of critical illness (such as severe pneumonia, ARDS, and shock), high levels of inflammatory cytokines and endogenous cortisol inhibit the enzyme 5'-deiodinase in peripheral tissues. Normally, this enzyme converts thyroxine (T4), a prohormone, into the biologically active triiodothyronine (T3). Because of this inhibition, T3 levels fall, while T4 is instead shunted toward the production of reverse T3 (an inactive metabolite). During the mild to moderate stages of euthyroid sick syndrome, TSH and free T4 levels remain strictly within normal limits. This physiological adaptation requires observation rather than hormone supplementation.

Step-by-Step Solution

1

Analyze the patient's clinical context.

The patient is critically ill in the intensive care unit with acute respiratory distress syndrome and septic shock.

Severe physical stress profoundly alters endocrine physiology, which must be considered before diagnosing a primary endocrine disorder.

2

Interpret the thyroid function test results.

TSH is normal, free T4 is normal, but total T3 is abnormally low.

This specific combination of normal central markers (TSH) and normal primary secretory product (T4) with low active hormone (T3) points to a peripheral metabolism issue.

3

Correlate the lab findings with the clinical context to reach a diagnosis.

The findings represent euthyroid sick syndrome (non-thyroidal illness syndrome), caused by reduced conversion of T4 to T3.

Identifying the underlying pathophysiologic mechanism prevents unnecessary and potentially harmful thyroid hormone supplementation in critically ill patients.

Key Concept

Euthyroid Sick Syndrome Pathophysiology

Question 1748Question →

A 65-year-old male with poorly controlled type 2 diabetes mellitus presents to the emergency department following 3 days of intractable vomiting, diffuse abdominal pain, and an inability to tolerate oral intake. He reports that he stopped taking his prescribed insulin regimen 48 hours ago due to his lack of appetite. On physical examination, he appears lethargic and mildly confused. His mucous membranes are completely dry, and he has markedly decreased skin turgor. His temperature is 37.1°C (98.8°F), blood pressure is 90/60 mm Hg, pulse is 112/min, and respiratory rate is 24/min.

Laboratory studies show the following:
Sodium: $142 \text{ mEq/L}$
Potassium: $3.8 \text{ mEq/L}$
Chloride: $94 \text{ mEq/L}$
Bicarbonate: $16 \text{ mEq/L}$
Blood urea nitrogen: $45 \text{ mg/dL}$
Serum creatinine: $2.2 \text{ mg/dL}$
Glucose: $550 \text{ mg/dL}$
Serum ketones: large

Assuming a normal serum anion gap of $12 \text{ mEq/L}$ and a normal serum bicarbonate of $24 \text{ mEq/L}$ , what is the delta ratio ( $\Delta\text{AG} / \Delta\text{HCO}_3^-$ ) for this patient?

Show answer & explanation

Answer: 2.5

Answer

2.5

The delta ratio (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) is calculated by first determining the anion gap, which is

142 - (94 + 16) = 32 \text{ mEq/L}

. The change in the anion gap (

\Delta\text{AG}

) is

32 - 12 = 20

. The change in bicarbonate (

\Delta\text{HCO}_3^-

) is

24 - 16 = 8

. Dividing the

\Delta\text{AG}

by the

\Delta\text{HCO}_3^-

gives

20 / 8 = 2.5

. In the clinical context of diabetic ketoacidosis (which produces a high anion gap metabolic acidosis), a delta ratio greater than

2.0

confirms the presence of an underlying concurrent metabolic alkalosis. This physiological state is perfectly consistent with this patient's history of severe, intractable vomiting, which causes a profound loss of gastric hydrochloric acid and volume contraction.

Step-by-Step Solution

1

Calculate the patient's anion gap (AG).

\text{AG} = 142 - (94 + 16) = 32 \text{ mEq/L}

The anion gap is calculated using the formula:

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

.

2

Calculate the delta anion gap (

\Delta\text{AG}

).

\Delta\text{AG} = 32 - 12 = 20

\Delta\text{AG}

is the calculated anion gap minus the normal expected anion gap (

12 \text{ mEq/L}

).

3

Calculate the delta bicarbonate (

\Delta\text{HCO}_3^-

).

\Delta\text{HCO}_3^- = 24 - 16 = 8

\Delta\text{HCO}_3^-

is the normal expected bicarbonate (

24 \text{ mEq/L}

) minus the patient's measured bicarbonate.

4

Calculate the delta ratio.

\Delta\text{Ratio} = \frac{20}{8} = 2.5

The delta ratio evaluates the relationship between the rise in unmeasured anions and the fall in bicarbonate. Dividing

\Delta\text{AG}

by

\Delta\text{HCO}_3^-

yields

2.5

.

Key Concept

In complex mixed acid-base disorders, calculating the delta ratio (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) helps uncover hidden metabolic derangements. A delta ratio

> 2.0

suggests a concurrent metabolic alkalosis superimposed on a high anion gap metabolic acidosis.

Estimated Time:1m 30s

Question 1749Question →

Match each of the following thyroid function test (TFT) laboratory profiles with the most likely underlying pathophysiologic diagnosis.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Elevated TSH with decreased free T4 matches Primary hypothyroidism. Suppressed TSH with elevated free T4 matches Primary hyperthyroidism. Elevated TSH with normal free T4 matches Subclinical hypothyroidism. Decreased TSH with decreased free T4 matches Central (secondary) hypothyroidism.

The physiological relationship between TSH and free T4 operates on a strict negative feedback loop. Primary thyroid disorders present with TSH and free T4 moving in opposite directions because the intact pituitary is responding appropriately to abnormal peripheral hormone levels. Central disorders present with them moving in the same direction, as the pituitary/hypothalamus is the source of the defect.

Step-by-Step Solution

1

Analyze the free T4 level to determine the clinical state of thyroid function.

Categorizes the patient into a hyperthyroid, hypothyroid, or euthyroid clinical state.

Free T4 represents the biologically active thyroid hormone in circulation.

2

Analyze the TSH level in relation to the free T4 level.

Determines whether the expected hypothalamic-pituitary-thyroid negative feedback loop is intact or disrupted.

If TSH and free T4 move in opposite directions, the defect is primary (at the thyroid). If they move in the same direction or TSH fails to respond properly, the defect is central (at the pituitary or hypothalamus).

Key Concept

Interpretation of the hypothalamic-pituitary-thyroid axis via classic TSH and free T4 patterns.

Question 1750Question →

A 68-year-old man is evaluated in the intensive care unit for worsening oxygenation and a new-onset fever. He was admitted 8 days ago for severe acute respiratory distress syndrome secondary to influenza A, requiring endotracheal intubation and mechanical ventilation. Over the past 48 hours, his fraction of inspired oxygen (FiO2) requirement has increased from 40% to 60%. His temperature is 39.1°C (102.4°F), blood pressure is 110/65 mm Hg, and pulse is 108/min. Physical examination reveals coarse rhonchi bilaterally and copious, purulent secretions from the endotracheal tube. A chest radiograph demonstrates a new consolidation in the left lower lobe. Endotracheal aspirate and blood cultures are obtained. The patient is empirically started on intravenous piperacillin-tazobactam, vancomycin, and amikacin. Three days later, his clinical condition has significantly improved, and his oxygen requirements have decreased. The endotracheal aspirate culture yields heavy growth of Pseudomonas aeruginosa. Susceptibility testing indicates the isolate is sensitive to piperacillin-tazobactam, ceftazidime, cefepime, and ciprofloxacin, but resistant to amikacin. Blood cultures show no growth after 72 hours. Which of the following is the most appropriate next step in pharmacotherapy?

Discontinue the current regimen and initiate therapy with intravenous ceftazidime and ciprofloxacin

Discontinue vancomycin and amikacin, and continue piperacillin-tazobactam monotherapy

Discontinue amikacin, and continue piperacillin-tazobactam and vancomycin

Continue the current empiric regimen of piperacillin-tazobactam, vancomycin, and amikacin

Discontinue vancomycin, and continue piperacillin-tazobactam and amikacin

Show answer & explanation

Answer: Discontinue vancomycin and amikacin, and continue piperacillin-tazobactam monotherapy

Answer

Discontinue vancomycin and amikacin, and continue piperacillin-tazobactam monotherapy

The core principle of antimicrobial stewardship in the intensive care unit is de-escalation. The patient's empiric regimen appropriately covered methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa. Once endotracheal aspirate cultures identified only Pseudomonas aeruginosa susceptible to piperacillin-tazobactam, MRSA coverage (vancomycin) must be discontinued. For definitive treatment of ventilator-associated pneumonia caused by Pseudomonas, a single active beta-lactam agent (in this case, piperacillin-tazobactam) is recommended. The addition of a second agent (such as an aminoglycoside or fluoroquinolone) is not indicated for definitive therapy unless the isolate is highly resistant to beta-lactams, and in this specific scenario, the isolate is resistant to the empirically chosen aminoglycoside anyway.

Step-by-Step Solution

1

Identify the clinical condition based on the presentation.

The patient developed a new fever, increased oxygen requirements, purulent secretions, and a new pulmonary infiltrate 8 days after intubation, consistent with a diagnosis of ventilator-associated pneumonia (VAP).

Establishing the diagnosis determines the required initial empiric antibiotic spectrum.

2

Evaluate the initial empiric therapy.

Empiric coverage appropriately included agents targeting MRSA (vancomycin) and providing double coverage for Pseudomonas aeruginosa (piperacillin-tazobactam and amikacin) given the patient's risk factors.

Current guidelines recommend broad initial coverage for VAP until causative organisms are identified.

3

Analyze the lower respiratory tract culture and susceptibility data.

The culture confirms Pseudomonas aeruginosa that is sensitive to piperacillin-tazobactam but resistant to amikacin. No MRSA was isolated.

Definitive therapy must be tailored based on microbiologic data to ensure efficacy and minimize toxicity.

4

Apply antimicrobial stewardship principles to modify the regimen.

Discontinue vancomycin (no MRSA isolated) and discontinue amikacin (isolate is resistant). Continue piperacillin-tazobactam as monotherapy.

For definitive treatment of VAP caused by Pseudomonas aeruginosa, de-escalation to a single active beta-lactam agent is standard practice. Double coverage is not routinely recommended for definitive therapy.

Key Concept

De-escalation of empiric broad-spectrum antibiotics to targeted monotherapy in ventilator-associated pneumonia once culture and susceptibility data are available.

Question 1751Question →

A 24-year-old woman with a history of type 1 diabetes mellitus presents to the emergency department with nausea, vomiting, and diffuse abdominal pain. She reports missing her insulin doses for the past two days due to illness. On physical examination, she appears lethargic. Her blood pressure is 105/65 mm Hg, heart rate is 118/min, and respiratory rate is 26/min with deep, rapid breaths. Laboratory studies show the following:

Laboratory test	Result
Serum sodium	140 mEq/L
Serum potassium	5.2 mEq/L
Serum chloride	98 mEq/L
Serum bicarbonate	12 mEq/L
Blood glucose	485 mg/dL
Serum creatinine	1.1 mg/dL

What is the patient's calculated serum anion gap? (Provide the numerical value in mEq/L).

Show answer & explanation

Answer: 30

Answer

30

The serum anion gap is calculated using the standard formula:

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

. In this patient, substituting the given laboratory values yields:

140 - (98 + 12) = 140 - 110 = 30

mEq/L. A normal anion gap is typically between 8 and 12 mEq/L. This patient has a significantly elevated anion gap of 30 mEq/L, which, in the context of her hyperglycemia and missed insulin doses, is highly consistent with diabetic ketoacidosis, a classic cause of high anion gap metabolic acidosis.

Step-by-Step Solution

1

Identify the standard formula for calculating the serum anion gap.

Anion Gap =

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

The anion gap is a calculated value used to differentiate the causes of metabolic acidosis by estimating the unmeasured anions in the serum.

2

Extract the relevant serum electrolyte values from the provided laboratory results.

Na^+ = 140

mEq/L,

Cl^- = 98

mEq/L, and

HCO_3^- = 12

mEq/L

These three specific electrolyte values are the components required for the standard anion gap calculation. Serum potassium is generally omitted from this formula on the USMLE.

3

Substitute the values into the formula and perform the calculation.

Anion Gap =

140 - (98 + 12) = 140 - 110 = 30

mEq/L

This mathematical operation yields the final numerical value of the patient's anion gap.

Key Concept

Calculation of the serum anion gap is a fundamental step in evaluating a patient with metabolic acidosis, utilizing the formula: Anion Gap =

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

.

Question 1752Question →

A 28-year-old individual with a history of type 1 diabetes mellitus presents to the acute care clinic with 48 hours of relentless nausea and forceful emesis. The patient reports being unable to retain oral fluids and consequently withheld their scheduled insulin doses. On physical assessment, the patient is lethargic with dry mucous membranes and delayed capillary refill. Vital signs show a temperature of 37.1°C (98.8°F), blood pressure of 96/58 mm Hg, heart rate of 115/min, and respiratory rate of 24/min.

Laboratory analysis reveals:
Serum sodium: 140 mEq/L
Serum potassium: 3.4 mEq/L
Serum chloride: 94 mEq/L
Serum bicarbonate: 16 mEq/L
Serum creatinine: 1.8 mg/dL
Blood glucose: 625 mg/dL

Arterial blood gas obtained on room air shows:
pH: 7.32
pCO₂: 32 mm Hg
pO₂: 94 mm Hg

Based on these findings, which of the following represents the most accurate characterization of this patient's acid-base status?

Elevated anion gap metabolic acidosis and concurrent metabolic alkalosis

Isolated elevated anion gap metabolic acidosis

Elevated anion gap metabolic acidosis and concurrent respiratory acidosis

Normal anion gap metabolic acidosis and concurrent metabolic alkalosis

Elevated anion gap metabolic acidosis and concurrent normal anion gap metabolic acidosis

Show answer & explanation

Answer: Elevated anion gap metabolic acidosis and concurrent metabolic alkalosis

Answer

Elevated anion gap metabolic acidosis and concurrent metabolic alkalosis

The correct answer accurately identifies the presence of two distinct metabolic disorders. The patient's initial laboratory values demonstrate acidemia (pH 7.32) and a low bicarbonate (16 mEq/L), indicating a primary metabolic acidosis. Calculating the anion gap yields

140 - (94 + 16) = 30

mEq/L, confirming an elevated anion gap metabolic acidosis, which is expected in diabetic ketoacidosis. To determine if respiratory compensation is adequate, Winter's formula is utilized (

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

). This calculation yields

32 \pm 2

mm Hg; the measured

pCO_2

is 32 mm Hg, confirming appropriate respiratory compensation without a primary respiratory disorder. Finally, the delta-delta gap is evaluated to uncover any hidden metabolic derangements. The excess anion gap is

30 - 12 = 18

. Adding this back to the measured bicarbonate yields an adjusted bicarbonate of

16 + 18 = 34

mEq/L. Because the adjusted bicarbonate is substantially greater than the normal value of 24 mEq/L, a concurrent metabolic alkalosis is diagnosed. This secondary alkalosis is clinically consistent with the patient's history of intractable vomiting and resultant hydrochloric acid loss.

Step-by-Step Solution

1

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

The

pH

is 7.32 (acidemia) and

HCO_3^-

is 16 mEq/L (low), indicating a primary metabolic acidosis.

Determining the primary disorder is the essential first step in acid-base analysis.

2

Calculate the anion gap to determine the etiology of the metabolic acidosis.

Anion gap =

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

=

140 - (94 + 16) = 30

mEq/L, which is elevated.

An elevated anion gap (> 12 mEq/L) points to the presence of unmeasured anions, such as ketones in diabetic ketoacidosis.

3

Apply Winter's formula to assess for a concurrent respiratory disorder.

Expected

pCO_2 = (1.5 \times 16) + 8 = 32 \pm 2

mm Hg. The measured

pCO_2

is exactly 32 mm Hg, meaning respiratory compensation is appropriate.

To determine if there is an independent respiratory acidosis or alkalosis occurring simultaneously.

4

Calculate the delta gap (Δ-Δ) to check for additional underlying metabolic disorders.

Delta gap = Calculated AG (30) - Normal AG (12) = 18. Adjusted

HCO_3^-

= Measured

HCO_3^-

(16) + Delta gap (18) = 34 mEq/L.

The delta gap reveals if there is a hidden normal anion gap metabolic acidosis or a concurrent metabolic alkalosis.

5

Interpret the adjusted bicarbonate level.

Since the adjusted bicarbonate (34 mEq/L) is significantly greater than the normal baseline of 24 mEq/L, a concurrent metabolic alkalosis is diagnosed.

The excess bicarbonate unmasked by the calculation confirms a secondary alkalotic process, consistent with the patient's severe vomiting.

Key Concept

Identification of a mixed metabolic disorder using the anion gap, Winter's formula, and the delta-delta gap calculations.

Estimated Time:1m 30s

Question 1753Question →

A 42-year-old woman presents to the clinic with progressive weight gain, easy bruising, and proximal muscle weakness over the past 8 months. Physical examination reveals facial plethora, prominent supraclavicular fat pads, and purple abdominal striae. Her blood pressure is 155/95 mmHg. The physician suspects Cushing syndrome and plans to initiate a diagnostic workup. Arrange the following diagnostic steps in the correct chronological sequence, from the initial action to the final localizing step.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct chronological sequence for diagnosing Cushing syndrome is to first exclude exogenous glucocorticoid exposure, followed by confirming endogenous hypercortisolism with initial screening tests, then measuring plasma ACTH to determine ACTH dependence, and finally performing a high-dose dexamethasone suppression test or pituitary MRI to localize the ACTH source.

The diagnostic evaluation of Cushing syndrome follows a strict stepwise approach to prevent diagnostic errors and unnecessary procedures. Exogenous glucocorticoid use is the most common etiology and must be ruled out clinically. Next, endogenous hypercortisolism is established using highly sensitive screening tests (e.g., 24-hour urine free cortisol, late-night salivary cortisol, or low-dose dexamethasone suppression test). Only after hypercortisolism is biochemically confirmed should early morning plasma ACTH be measured to classify the condition as ACTH-dependent or ACTH-independent. Finally, if the condition is ACTH-dependent, specialized testing (such as a high-dose dexamethasone suppression test, CRH stimulation test, or pituitary MRI) is utilized to differentiate a pituitary source (Cushing disease) from an ectopic source.

Step-by-Step Solution

1

Exclude exogenous glucocorticoid exposure.

Rules out iatrogenic Cushing syndrome.

Exogenous glucocorticoids are the most common cause of Cushingoid features and require no further biochemical evaluation if identified.

2

Perform initial biochemical screening (e.g., 24-hour urine free cortisol).

Confirms the presence of endogenous hypercortisolism.

Pathological hypercortisolism must be definitively proven before investigating its specific source to avoid false-positive diagnostic cascades.

3

Measure early morning plasma ACTH.

Categorizes the syndrome as ACTH-dependent or ACTH-independent.

A suppressed ACTH level points to an adrenal tumor, whereas a normal or elevated ACTH level points to a pituitary or ectopic source.

4

Perform high-dose dexamethasone suppression test or pituitary MRI.

Localizes the ACTH-dependent source.

This is necessary to differentiate a pituitary adenoma (Cushing disease), which typically suppresses with high-dose dexamethasone, from ectopic ACTH secretion, which typically does not.

Key Concept

Diagnostic algorithm for Cushing syndrome

Question 1754Question →

Match each of the following comprehensive thyroid function laboratory profiles to the most likely underlying pathophysiological diagnosis. Assume all patients are adult individuals without current acute severe illness unless the laboratory pattern specifically indicates otherwise.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The laboratory profiles distinctly match Exogenous levothyroxine administration, Resistance to thyroid hormone, Thyrotropin-secreting pituitary adenoma, Thyroxine-binding globulin excess, and Euthyroid sick syndrome, based on an evaluation of central feedback loops and peripheral metabolic markers.

Each laboratory profile presents a pathognomonic endocrine pattern. Exogenous levothyroxine shows thyrotoxicosis with completely suppressed endogenous production (low Tg, low RAIU). Resistance to thyroid hormone and TSH-omas both display high FT4 and unsuppressed TSH, but are definitively separated by the alpha-subunit ratio and anatomic imaging. TBG excess elevates total T4 but leaves active free T4 normal while lowering T3 resin uptake. Finally, euthyroid sick syndrome shifts T4 metabolism from the active T3 pathway into the inactive reverse T3 pathway without altering free T4 or TSH.

Step-by-Step Solution

1

Analyze the core hypothalamic-pituitary-thyroid axis using TSH and Free T4.

Identifies if the primary defect is at the level of the thyroid gland (classic inverse relationship) or central/systemic (unsuppressed or discordant relationship).

Establishes the foundational distinction between primary and central/peripheral thyroid pathology.

2

Evaluate secondary diagnostic markers for profiles exhibiting suppressed TSH and high Free T4.

RAIU and thyroglobulin differentiate exogenous hormone intake (both values are low) from destructive thyroiditis (low RAIU, high thyroglobulin) or Graves disease (high RAIU).

Determines the exact etiology of primary thyrotoxicosis when standard labs are insufficient.

3

Distinguish between etiologies presenting with unsuppressed TSH and high Free T4.

Alpha-subunit to TSH molar ratio and pituitary MRI separate a TSH-secreting adenoma (both are abnormal) from resistance to thyroid hormone (both are normal).

Resolves the complex differential diagnosis for secondary hyperthyroid laboratory patterns.

4

Assess peripheral metabolism and binding capacity for isolated Total T4 or Total T3 abnormalities.

Total T4, T3 resin uptake, and reverse T3 accurately identify TBG excess (high Total T4, low resin uptake) and euthyroid sick syndrome (low T3, high reverse T3).

Explains isolated laboratory abnormalities when the metabolically active Free T4 and central TSH remain normal.

Key Concept

Advanced interpretation of thyroid function tests integrating central feedback loops, peripheral binding proteins, and alternate metabolic pathways.

Question 1755Question →

A 42-year-old woman comes to the emergency department complaining of profound muscle weakness and generalized fatigue that has worsened over the past two weeks. She reports intermittent difficulty swallowing dry foods and feeling like she constantly has "sand in her eyes." Her past medical history is unremarkable, and she takes no medications. Vital signs are within normal limits.

Laboratory studies are obtained and reveal the following:

Test	Result
Serum sodium	$139 \text{ mEq/L}$
Serum potassium	$2.1 \text{ mEq/L}$
Serum chloride	$114 \text{ mEq/L}$
Serum bicarbonate	$15 \text{ mEq/L}$
Blood urea nitrogen	$12 \text{ mg/dL}$
Serum creatinine	$0.9 \text{ mg/dL}$
Urine sodium	$45 \text{ mEq/L}$
Urine potassium	$35 \text{ mEq/L}$
Urine chloride	$55 \text{ mEq/L}$
Urine pH	$6.8$

Which of the following is the primary pathophysiologic mechanism underlying this patient's electrolyte abnormalities?

Impaired hydrogen ion secretion by the alpha-intercalated cells

Decreased bicarbonate reabsorption in the proximal convoluted tubule

Gastrointestinal loss of bicarbonate-rich secretions

Reduced sensitivity of the principal cells to aldosterone

Defective sodium-potassium-chloride cotransport in the loop of Henle

Show answer & explanation

Answer: Impaired hydrogen ion secretion by the alpha-intercalated cells

Answer

Impaired hydrogen ion secretion by the alpha-intercalated cells

The correct answer accurately describes the pathophysiology of Type 1 (distal) Renal Tubular Acidosis (RTA). This patient presents with a normal anion gap metabolic acidosis (NAGMA), profound hypokalemia, and a positive urine anion gap (

+25 \text{ mEq/L}

), which definitively isolates the kidney as the source of the acidosis due to impaired ammonium excretion. Furthermore, the urine pH is inappropriately alkaline (

6.8

), which is the hallmark of Type 1 RTA. Type 1 RTA results from the inability of the alpha-intercalated cells in the distal nephron to secrete hydrogen ions (

H^+

). The patient's symptoms of dry mouth and dry eyes strongly suggest Sjögren syndrome, which is a classic autoimmune etiology for Type 1 RTA.

Step-by-Step Solution

1

Calculate the serum anion gap to classify the metabolic acidosis.

\text{Serum AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 139 - (114 + 15) = 10 \text{ mEq/L}

.

Determines whether the acidosis is driven by unmeasured anions (high AG) or loss of bicarbonate (normal AG). A value of

10

indicates a normal anion gap metabolic acidosis (NAGMA).

2

Calculate the Urine Anion Gap (UAG) to distinguish between gastrointestinal and renal causes of NAGMA.

\text{UAG} = (\text{Urine Na}^+ + \text{Urine K}^+) - \text{Urine Cl}^- = (45 + 35) - 55 = +25 \text{ mEq/L}

.

A negative UAG suggests appropriate renal excretion of

NH_4Cl

(typical of GI bicarbonate losses like diarrhea). A positive UAG indicates impaired renal

NH_4^+

excretion, pointing to a Renal Tubular Acidosis (RTA).

3

Evaluate the serum potassium and urine pH to differentiate the specific type of RTA.

The patient has severe hypokalemia (

2.1 \text{ mEq/L}

) and an inappropriately alkaline urine (pH

6.8

).

Hypokalemia rules out Type 4 RTA (which features hyperkalemia). An alkaline urine pH (

> 5.5

) in the setting of severe systemic acidosis is the hallmark of Type 1 (distal) RTA. In Type 2 RTA, the distal tubule is intact and urine pH is typically

< 5.5

once the serum bicarbonate reaches a low steady state.

4

Correlate the diagnostic findings and clinical presentation to the underlying cellular mechanism.

The patient's "sand in eyes" and dry mouth indicate Sjögren syndrome, a classic autoimmune trigger for Type 1 RTA.

Type 1 RTA is caused by a defect in

H^+

secretion by the alpha-intercalated cells of the distal tubule and collecting duct, leading to systemic acidosis, compensatory hypokalemia, and failure to acidify urine.

Key Concept

The pathophysiology and diagnostic differentiation of Renal Tubular Acidosis (RTA) using the Urine Anion Gap, serum potassium, and urine pH.

Alternative Method

Memorize the 'RTA potassium rule': Type 4 RTA is the only type with hyperkalemia. Between Type 1 and Type 2 (which both have hypokalemia), only Type 1 is rigidly incapable of acidifying the urine (pH always > 5.5). Coupling this with the 'sicca' symptoms (Sjögren's) directly points to Type 1.

Estimated Time:3m 0s

Question 1756Question →

A 38-year-old woman comes to the outpatient clinic due to worsening fatigue, weight gain, and cold intolerance over the last 4 months. She also reports that she has not had a menstrual period for 6 months. Her past medical history is notable for a severe postpartum hemorrhage 4 years ago requiring a blood transfusion. Physical examination reveals dry skin and delayed relaxation of the deep tendon reflexes. Laboratory evaluation shows:

Free T4: $0.4\text{ ng/dL}$ (normal: $0.9-1.7\text{ ng/dL}$ )
TSH: $1.2\text{ }\mu\text{U/mL}$ (normal: $0.5-5.0\text{ }\mu\text{U/mL}$ )

Based on these findings, which of the following is the most likely diagnosis?

Central hypothyroidism

Primary hypothyroidism

Subclinical hypothyroidism

Euthyroid sick syndrome

Resistance to thyroid hormone

Show answer & explanation

Answer: Central hypothyroidism

Answer

Central hypothyroidism

The correct diagnosis is central hypothyroidism. The patient presents with classic symptoms of hypothyroidism (fatigue, cold intolerance, weight gain, delayed reflexes). Her laboratory results show a low free T4, confirming a hypothyroid state. However, her TSH is within the normal range. In primary thyroid failure, a low free T4 removes negative feedback on the pituitary gland, leading to a significantly elevated TSH. An inappropriately normal or low TSH in the presence of a low free T4 indicates a failure of the pituitary gland or hypothalamus to respond properly, known as central (or secondary/tertiary) hypothyroidism. Her history of a severe postpartum hemorrhage requiring a transfusion strongly suggests Sheehan syndrome (ischemic necrosis of the pituitary gland), which is a classic cause of central hypothyroidism and panhypopituitarism (accounting for her amenorrhea).

Step-by-Step Solution

1

Interpret the free T4 level.

Free T4 is below the normal range (

0.4\text{ ng/dL}

).

Establishes that the patient is in a hypothyroid state, which matches her symptoms of fatigue, weight gain, and cold intolerance.

2

Evaluate the TSH level in the context of the low free T4.

TSH is

1.2\text{ }\mu\text{U/mL}

, which is within the normal reference range.

Determines whether the hypothyroidism is primary or central. In a normal physiologic state, a low free T4 should stimulate the pituitary to secrete high amounts of TSH. An inappropriately normal TSH indicates central axis failure.

3

Correlate laboratory findings with the patient's medical history.

The history of severe postpartum hemorrhage suggests Sheehan syndrome (postpartum pituitary necrosis).

Confirms the diagnosis of a pituitary disorder (central hypothyroidism), which also explains her amenorrhea due to a concurrent loss of gonadotropins.

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 level with an inappropriately normal or low TSH level, often caused by pituitary pathology.

Question 1757Question →

Match each of the following patient clinical presentations with the most likely corresponding renal biopsy findings on light, immunofluorescence, or electron microscopy.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Solid tumor matches subepithelial deposits (Membranous nephropathy); Hepatitis C matches subendothelial deposits (Type I MPGN); Pulmonary-renal syndrome with normal complement matches fibrinoid necrosis and crescents without immunoglobulins (Pauci-immune GN); and Hearing loss with lenticonus matches lamellation of the lamina densa (Alport syndrome).

Each matching pair requires linking a classic systemic presentation to its definitive histologic findings. Membranous nephropathy (subepithelial deposits) is strongly tied to solid tumors. Type I MPGN (subendothelial deposits with double contours) is linked to Hepatitis C and cryoglobulinemia. Pauci-immune crescentic GN (crescents without immunoglobulin) is identified by pulmonary-renal symptoms without immune complex consumption (evidenced by normal complement). Alport syndrome (GBM splitting) presents with type IV collagen defects uniquely affecting the ears, eyes, and kidneys.

Step-by-Step Solution

1

Analyze the vignette of the 55-year-old man with solid organ malignancy and nephrotic syndrome.

Identified as secondary membranous nephropathy.

Solid tumors are classic secondary causes of membranous nephropathy, which presents with isolated massive proteinuria and pathologically shows subepithelial 'spike and dome' deposits.

2

Analyze the vignette of the 48-year-old woman with Hepatitis C, purpura, and a mixed urinary sediment.

Identified as cryoglobulinemia-associated Membranoproliferative Glomerulonephritis (MPGN).

HCV drives cryoglobulin production, which deposits in the subendothelial space, causing a 'tram-track' appearance on microscopy and hypocomplementemia.

3

Evaluate the 62-year-old man with pulmonary-renal syndrome, normal complement, and negative anti-GBM antibodies.

Identified as Pauci-immune crescentic glomerulonephritis (likely ANCA vasculitis).

Normal complement and negative anti-GBM effectively rule out Goodpasture syndrome and lupus/post-infectious glomerulonephritis, pointing strictly to pauci-immune crescentic GN.

4

Assess the 25-year-old man with hearing loss, lenticonus, and hematuria.

Identified as Alport syndrome.

Mutations in type IV collagen lead to sensorineural deafness, specific eye abnormalities, and characteristic splitting/lamellation of the glomerular basement membrane on electron microscopy.

Key Concept

Correlation of systemic clinical syndromes (malignancy, chronic infections, vasculitis, genetic disorders) with their specific glomerular histopathologic and electron microscopic biopsy patterns.

Question 1758Question →

A 48-year-old man comes to the clinic reporting progressive fatigue, weight gain, and decreased libido over the past six months. He also mentions experiencing intermittent frontal headaches and notes that he frequently bumps into doorframes on his left and right sides. Vital signs include a heart rate of 58/min and a blood pressure of 110/70 mm Hg. Physical examination is notable for delayed relaxation of deep tendon reflexes. Visual field testing by confrontation reveals a loss of peripheral vision in both eyes.

Laboratory studies show the following:

Test	Result	Normal Range
Thyroid-stimulating hormone (TSH)	0.7 µU/mL	0.4-4.0 µU/mL
Free thyroxine (T4)	0.3 ng/dL	0.8-1.8 ng/dL

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

Initiate levothyroxine monotherapy

Perform an ultrasound of the thyroid gland

Obtain a magnetic resonance imaging of the brain

Measure serum thyroid peroxidase antibodies

Measure serum reverse triiodothyronine (T3)

Show answer & explanation

Answer: Obtain a magnetic resonance imaging of the brain

Answer

The most appropriate next step is to obtain a magnetic resonance imaging (MRI) of the brain.

The correct action is to obtain a magnetic resonance imaging of the brain. This patient presents with symptoms of hypothyroidism (fatigue, weight gain, delayed reflexes) alongside headaches, decreased libido, and bitemporal hemianopsia. The laboratory findings of a low free T4 with an inappropriately normal TSH confirm secondary (central) hypothyroidism rather than a primary thyroid disorder. The combination of hypopituitarism and optic chiasm compression (bitemporal hemianopsia) is classic for a pituitary macroadenoma. The most critical next step to evaluate a suspected pituitary mass and determine the extent of anatomical compression is an MRI of the brain. Note that while evaluating the adrenal axis (morning cortisol) is also required before initiating thyroid hormone replacement to prevent an adrenal crisis, neuroimaging is definitively indicated to assess the neurologic threat posed by the expanding mass.

Step-by-Step Solution

1

Interpret the thyroid function tests.

The patient has a low free T4 level along with an inappropriately normal TSH level.

In primary hypothyroidism, a low T4 would stimulate the anterior pituitary to secrete high levels of TSH. A normal or low TSH in the setting of low T4 confirms secondary (central) hypothyroidism due to pituitary or hypothalamic dysfunction.

2

Synthesize the biochemical findings with the clinical presentation.

The combination of secondary hypothyroidism, symptoms of hypogonadism (decreased libido), and signs of optic chiasm compression (bitemporal hemianopsia and headaches) points to a pituitary mass.

Pituitary macroadenomas (benign tumors >10 mm) can expand superiorly to compress the optic chiasm, causing bilateral loss of peripheral vision, and can disrupt normal anterior pituitary function, leading to hypopituitarism.

3

Determine the most appropriate next step in management based on the synthesized diagnosis.

Obtain a magnetic resonance imaging (MRI) of the brain.

MRI is the gold standard imaging modality for visualizing the sella turcica, confirming the presence of a pituitary adenoma, and evaluating the extent of optic chiasm compression before considering surgical or medical interventions.

Key Concept

Secondary hypothyroidism (low free T4 with a normal or low TSH) accompanied by bitemporal hemianopsia is highly indicative of a pituitary macroadenoma, which requires an MRI of the brain for definitive diagnosis and evaluation of mass effect.

Question 1759Question →

A $42$ -year-old man is admitted to the hospital with newly diagnosed Burkitt lymphoma. He has a massive tumor burden and is scheduled to receive intensive induction chemotherapy. Baseline laboratory studies reveal a serum uric acid of $10.2$ mg/dL, potassium of $5.1$ mEq/L, phosphorus of $4.8$ mg/dL, and creatinine of $1.3$ mg/dL.

Arrange the following steps in the most appropriate chronological order for the prevention and stepwise management of tumor lysis syndrome in this patient.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct chronological sequence is: (1) Initiate prophylaxis with hydration and a hypouricemic agent, (2) Commence chemotherapy, (3) Perform serial laboratory monitoring, (4) Administer targeted medical therapy for electrolyte derangements, and (5) Initiate hemodialysis if medical management fails.

The management of patients at high risk for tumor lysis syndrome follows a strict chronological algorithm to prevent severe morbidity from massive intracellular constituent release. The sequence mandates pre-medication (aggressive hydration and hypouricemic agents) to optimize renal clearance, followed by the inciting event (chemotherapy). Subsequent care relies on aggressive serial monitoring, escalating to targeted medical therapy for initial derangements, and reserving hemodialysis for refractory, life-threatening cases.

Step-by-Step Solution

1

Administer intravenous hydration and a hypouricemic agent (e.g., rasburicase).

Lowers serum uric acid and increases renal tubular flow.

Prevents the precipitation of uric acid and calcium phosphate crystals in the renal tubules, which is the primary driver of acute kidney injury in TLS.

2

Initiate cytotoxic chemotherapy.

Begins tumor cell destruction.

Treats the primary aggressive malignancy (Burkitt lymphoma) only after the patient is adequately protected against the metabolic consequences of rapid cell lysis.

3

Monitor electrolytes and renal function every 4 to 6 hours.

Detects hyperkalemia, hyperphosphatemia, hypocalcemia, and hyperuricemia.

Early detection of evolving TLS allows for prompt, step-wise medical intervention before complications become fatal.

4

Provide targeted medical therapy (e.g., intravenous calcium, insulin/dextrose).

Stabilizes the cardiac membrane and shifts potassium intracellularly.

Prevents fatal arrhythmias and seizures resulting from acute electrolyte shifts during massive cell lysis.

5

Consult nephrology for hemodialysis.

Mechanically removes excess potassium, phosphate, and uric acid while managing volume status.

Serves as the ultimate salvage therapy for cases of acute kidney injury or metabolic derangements that are refractory to standard medical management.

Key Concept

Prevention and sequential management of Tumor Lysis Syndrome (TLS) in high-risk hematologic malignancies.

Question 1760Question →

Immunosuppressive and disease-modifying therapies used in rheumatologic conditions are associated with distinct adverse effect profiles requiring specific monitoring strategies. Match each medication below with its most characteristic adverse effect profile.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Hydroxychloroquine matches with macular toxicity; Cyclophosphamide matches with acrolein-mediated hemorrhagic cystitis; Infliximab matches with reactivation of latent tuberculosis; Methotrexate matches with hepatic fibrosis and oral ulcers.

The correct pairings align each rheumatologic medication with its most distinct and highly tested adverse effect, which directly informs the required clinical monitoring (e.g., ophthalmologic exams for hydroxychloroquine, TB testing for infliximab, mesna for cyclophosphamide, and folate supplementation for methotrexate).

Step-by-Step Solution

1

Analyze the adverse effect profile of hydroxychloroquine.

Identified as a cause of irreversible retinal toxicity, matching it to the requirement for periodic visual field testing.

Ophthalmologic screening is the standard of care for patients on long-term hydroxychloroquine.

2

Analyze the adverse effect profile of cyclophosphamide.

Identified as a cause of hemorrhagic cystitis and bladder cancer due to its metabolite acrolein.

Understanding the metabolism of cyclophosphamide explains its specific urothelial toxicity.

3

Analyze the adverse effect profile of infliximab.

Identified as a TNF-alpha inhibitor that impairs granuloma maintenance, matching it to latent tuberculosis reactivation.

TNF-alpha is mechanistically required to contain mycobacterial infections.

4

Analyze the adverse effect profile of methotrexate.

Identified as causing hepatotoxicity and stomatitis, which are mitigated by folic acid.

Methotrexate's mechanism as a folate antimetabolite directly leads to these toxicities.

Key Concept

Adverse effect profiles and monitoring requirements of disease-modifying antirheumatic drugs (DMARDs) and biologic agents.

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