A 35-year-old female presents with persistent daytime fatigue, muscle weakness, and weight loss. On examination, she exhibits diffuse hyperpigmentation of palmar creases and buccal mucosa. Blood pressure is 88/54 mmHg. Serum electrolytes reveal sodium 128 mmol/L and potassium 5.6 mmol/L. What is the most definitive diagnostic test to establish primary adrenal insufficiency (Addison's disease)?

Answer and explanation

Correct answer: C. Cosyntropin (synthetic ACTH) stimulation test

  1. Step 1: Recognize signs: Hyperpigmentation (elevated POMC/ACTH), hypotension, hyponatremia, and hyperkalemia point to primary adrenocortical failure.
  2. Step 2: The standard diagnostic test is the rapid ACTH stimulation test (cosyntropin 250 mcg IV/IM): failure of serum cortisol to rise above 18 mcg/dL (500 nmol/L) at 30 or 60 minutes confirms adrenal insufficiency.
Answer
Cosyntropin (synthetic ACTH) stimulation test.
Why not the others: 'Random aldosterone' fluctuates widely with posture and volume and cannot diagnose insufficiency; '24-hour urinary free cortisol' is used to screen for Cushing's syndrome (hypercortisolemia); 'Insulin tolerance test' carries seizure and cardiovascular risks and is reserved for specialized hypothalamic-pituitary testing.

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