A 2-week-old full-term male presents with failure to thrive, lethargy, vomiting, and profound dehydration. Serum sodium is 118 mEq/L, serum potassium is 7.8 mEq/L, and blood glucose is 55 mg/dL. External genitalia show hyperpigmented scrotum with descended testes. What is the most likely diagnosis and diagnostic enzyme deficiency?

Answer and explanation

Correct answer: B. Congenital adrenal hyperplasia; 21-hydroxylase deficiency

  1. Step 1: Neonatal salt-wasting crisis (hyponatremia, hyperkalemia, hypoglycemia, hypovolemic shock) with scrotal hyperpigmentation is classic for classic salt-wasting Congenital Adrenal Hyperplasia (CAH).
  2. Step 2: Over 90% of CAH cases result from 21-hydroxylase deficiency, leading to deficient cortisol and aldosterone synthesis with excess adrenal androgens.
  3. Step 3: 11-beta-hydroxylase deficiency presents with hypertension and hypokalemia; pyloric stenosis causes hypochloremic metabolic alkalosis.

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