A 52-year-old man from Delhi presents with a 3-month history of progressive fatigue, muscle weakness, and polyuria. On examination, blood pressure is 165/105 mmHg. Laboratory investigations reveal: serum potassium 2.8 mEq/L, serum sodium 148 mEq/L, serum bicarbonate 32 mEq/L, and plasma renin activity is suppressed at 0.3 ng/mL/hr (normal 0.5–1.6). Plasma aldosterone is elevated at 28 ng/dL (normal 4–31, but inappropriately high for the sodium level). Abdominal CT shows a 2 cm left adrenal nodule. Which mechanism best explains the hypokalemia and metabolic alkalosis in this patient?

Explanation

## Pathophysiology of Primary Hyperaldosteronism **Key Point:** This patient has primary hyperaldosteronism (Conn syndrome) from an adrenal adenoma, characterized by autonomous aldosterone secretion independent of renin-angiotensin signaling. ### Mechanism of Hypokalemia and Metabolic Alkalosis Aldosterone acts on the principal cells of the collecting duct via the mineralocorticoid receptor: 1. **Potassium wasting:** Aldosterone increases the expression and activity of the epithelial sodium channel (ENaC) and the renal outer medullary potassium (ROMK) channel, promoting Na^+^ reabsorption and K^+^ secretion. 2. **Hydrogen ion retention:** Aldosterone simultaneously stimulates H^+^ secretion via the H^+^-ATPase in intercalated cells, leading to metabolic alkalosis. 3. **Suppressed renin:** The volume expansion from sodium retention and hypertension suppresses plasma renin activity via negative feedback at the juxtaglomerular apparatus. ### Diagnostic Clues in This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Plasma renin activity** | 0.3 ng/mL/hr (suppressed) | Rules out secondary hyperaldosteronism (which would have elevated renin) | | **Plasma aldosterone** | 28 ng/dL (inappropriately high despite high Na^+^) | Autonomous production; aldosterone should be suppressed when Na^+^ is elevated | | **Aldosterone-to-renin ratio** | ~93 (markedly elevated; normal <30) | Diagnostic for primary hyperaldosteronism | | **Adrenal nodule** | 2 cm on CT | Consistent with aldosterone-producing adenoma (APA) | **High-Yield:** The combination of **suppressed renin + elevated aldosterone + hypokalemia + metabolic alkalosis + adrenal nodule** is pathognomonic for primary hyperaldosteronism. **Clinical Pearl:** Hypokalemia in primary hyperaldosteronism is due to **increased distal K^+^ secretion**, not reduced intake or GI losses. The metabolic alkalosis occurs because H^+^ secretion is coupled to Na^+^ reabsorption; as aldosterone drives Na^+^ reabsorption, H^+^ excretion increases proportionally.