A 38-year-old woman presents with hypertension (BP 165/105 mmHg), hypokalemia (K⁺ 2.8 mEq/L), and metabolic alkalosis (pH 7.48, HCO₃⁻ 32 mEq/L). Plasma aldosterone is markedly elevated (45 ng/dL; normal <15) with suppressed plasma renin activity (0.2 ng/mL/hr; normal 0.5–2.5). A contralateral adrenal CT shows a 1.2 cm nodule. Which feature best distinguishes primary aldosteronism from secondary aldosteronism?

Explanation

## Distinguishing Primary from Secondary Aldosteronism ### Key Pathophysiologic Difference **Key Point:** The hallmark discriminator between primary and secondary aldosteronism is the **plasma renin activity (PRA) response to aldosterone elevation**. - **Primary aldosteronism**: Autonomous aldosterone secretion from the adrenal gland → suppresses renin via negative feedback → **low PRA despite high aldosterone** - **Secondary aldosteronism**: Aldosterone is appropriately elevated in response to activation of the renin–angiotensin–aldosterone system (RAAS) → **high PRA and high aldosterone together** ### Comparative Table | Feature | Primary Aldosteronism | Secondary Aldosteronism | | --- | --- | --- | | **Plasma Aldosterone** | ↑↑ (>15 ng/dL) | ↑ (elevated) | | **Plasma Renin Activity** | ↓↓ (suppressed <0.5) | ↑↑ (elevated) | | **Aldosterone/Renin Ratio** | >20–30 (diagnostic) | <10 | | **Blood Pressure** | Usually elevated | Variable (depends on cause) | | **Cause** | Adrenal adenoma, bilateral hyperplasia | Heart failure, cirrhosis, renal artery stenosis, nephrotic syndrome | | **Mechanism** | Autonomous secretion | Appropriate response to RAAS activation | ### Clinical Pearl **Clinical Pearl:** The **aldosterone-to-renin ratio (ARR)** is the screening test of choice for primary aldosteronism. A ratio >20 (using ng/dL and ng/mL/hr units) with suppressed PRA is diagnostic. In this case, the ARR = 45 ÷ 0.2 = 225, which is pathognomonic for primary aldosteronism. ### High-Yield Summary **High-Yield:** Remember the **inverse relationship** in primary aldosteronism: ↑ aldosterone + ↓ renin = **autonomous adrenal disease**. Conversely, ↑ aldosterone + ↑ renin = **secondary (appropriate) response**. ### Why Other Options Are Misleading - **Adrenal nodule presence**: While common in primary aldosteronism, nodules can be incidental and non-functional. Secondary aldosteronism does not produce adrenal nodules, but the absence of a nodule does not exclude primary aldosteronism (bilateral hyperplasia accounts for ~40% of cases). - **Severity of hypokalemia/alkalosis**: Both conditions can present with similar degrees of electrolyte derangement; severity is not discriminatory. - **Elevated urinary potassium**: Both conditions cause urinary potassium wasting due to aldosterone-mediated renal effects; this is not distinctive.