**Core Concept:**
The captopril test is a pharmacological maneuver used in the evaluation of angiotensin-converting enzyme (ACE) inhibitors' effects on aldosterone secretion in patients with primary aldosteronism (Conn's syndrome). ACE inhibitors block the effects of angiotensin II, a potent stimulator of aldosterone secretion.

**Why the Correct Answer is Right:**
The correct answer is **D.** Captopril, an ACE inhibitor, is administered to patients with suspected primary aldosteronism (Conn's syndrome). The test works by blocking the effects of angiotensin II, which stimulates aldosterone production. After captopril administration, a decrease in plasma renin activity (PRA) and aldosterone levels is expected. This is due to the inhibition of angiotensin II, which is a potent stimulator of aldosterone secretion.

**Why Each Wrong Option is Incorrect:**
A. **Option A:** Angiotensin II antagonists (e.g., losartan) are incorrect for the captopril test. These drugs block the effects of angiotensin II, which would not lead to an expected decrease in aldosterone levels after administration.

B. **Option B:** Aldosterone antagonists (e.g., eplerenone) are also incorrect for the captopril test. These drugs block the effects of aldosterone, which would not lead to an expected decrease in aldosterone levels after administration.

C. **Option C:** Mineralocorticoid receptor antagonists (e.g., fludrocortisone) are also incorrect for the captopril test. These drugs affect aldosterone receptors and block aldosterone's effects, not angiotensin II.

**Clinical Pearl:**
The captopril test is a valuable tool in the diagnosis of primary aldosteronism (Conn's syndrome), as it helps differentiate between aldosterone-secreting adenomas and bilateral adrenal hyperplasia (Conn's syndrome). In patients with primary aldosteronism, administration of captopril results in a decrease in plasma renin activity and aldosterone levels, which is not observed in other etiologies of hyperaldosteronism.

**Why the Correct Answer is D:**
In patients with primary aldosteronism, administration of captopril results in a decrease in plasma renin activity and aldosterone levels. This is due to the blocking of angiotensin II, which is a potent stimulator of aldosterone secretion. In contrast, patients with other etiologies of hyperaldosteronism (e.g., bilateral adrenal hyperplasia) do not exhibit a decrease in plasma renin activity and aldosterone levels after captopril administration. This test aids in differentiating between aldosterone-secreting adenomas and bilateral adrenal hyperplasia (Conn's syndrome) in patients with primary aldosteronism.