In distinguishing primary from secondary adrenal insufficiency, the ACTH stimulation test shows which result in primary adrenal insufficiency?

• A. Cortisol rises normally after ACTH stimulation
• B. Cortisol fails to rise after ACTH stimulation
• C. Decreased cortisol production independent of ACTH
• D. Increased urinary cortisol after metyrapone

Answer: (B)

The key idea is that the ACTH stimulation test checks whether the adrenal cortex can produce cortisol when driven by ACTH. In primary adrenal insufficiency, the adrenal glands themselves are damaged and cannot respond to ACTH, so cortisol does not rise after stimulation (the peak remains inappropriately low). This blunted response is what distinguishes primary adrenal failure from secondary causes, where the adrenals are intact and cortisol would increase in response to the ACTH given. The other options describe either a normal adrenal response or refer to a different test or outcome that doesn't reflect the expected primary adrenal insufficiency pattern.