Match each patient's presenting clinical and laboratory profile with the most likely under… — USMLE Step 2 CK

Match each patient's presenting clinical and laboratory profile with the most likely underlying pathophysiologic condition.

Blood pressure $168/98$ mmHg. Serum $\text{Na}^+$ $149$ mEq/L, $\text{K}^+$ $2.7$ mEq/L, $\text{HCO}_3^-$ $33$ mEq/L.Primary hyperaldosteronism
Blood pressure $88/54$ mmHg. Serum $\text{Na}^+$ $129$ mEq/L, $\text{K}^+$ $5.9$ mEq/L, $\text{HCO}_3^-$ $17$ mEq/L.Primary adrenal insufficiency
Blood pressure $118/76$ mmHg. Serum $\text{Na}^+$ $121$ mEq/L, $\text{K}^+$ $4.1$ mEq/L, Serum osmolality $252$ mOsm/kg, Urine osmolality $540$ mOsm/kg.Syndrome of inappropriate antidiuretic hormone (SIADH)
Blood pressure $112/70$ mmHg. Serum $\text{Na}^+$ $153$ mEq/L, $\text{K}^+$ $4.1$ mEq/L, Serum osmolality $315$ mOsm/kg, Urine osmolality $145$ mOsm/kg.Diabetes insipidus

Answer

The profiles correspond to Primary hyperaldosteronism, Primary adrenal insufficiency, SIADH, and Diabetes insipidus, respectively.

Each laboratory profile distinctly reflects a specific hormonal axis that governs renal handling of sodium, potassium, free water, and hydrogen ions.

Step-by-Step Solution

Analyze the first profile for volume, potassium, and acid-base status.

Hypertension, hypokalemia, and metabolic alkalosis are identified. This matches Primary hyperaldosteronism.

Aldosterone stimulates ENaC channels for sodium reabsorption while increasing potassium and proton excretion.

Evaluate the second profile for missing hormonal effects.

Hypotension, hyponatremia, hyperkalemia, and metabolic acidosis are present. This matches Primary adrenal insufficiency.

The complete absence of aldosterone reverses the typical distal tubule exchanges, leading to sodium loss and potassium/hydrogen retention.

Assess the third profile, focusing on serum and urine osmolalities.

Euvolemic hypotonic hyponatremia with a concentrated urine (urine osmolality > 100 mOsm/kg) is identified. This matches SIADH.

Excess ADH causes pure water retention, diluting serum sodium while concentrating the urine.

Examine the fourth profile's water balance.

Hypernatremia with hyperosmolality and an inappropriately dilute urine is identified. This matches Diabetes insipidus.

Lack of ADH prevents water reabsorption, leading to excessive free water excretion and subsequent concentration of serum electrolytes.

Key Concept

Integration of patient volume status, serum sodium, potassium levels, and acid-base parameters to differentiate underlying renal tubular and endocrine disorders.

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