**Core Concept**
Conn syndrome, also known as primary aldosteronism, is a condition characterized by excessive secretion of aldosterone, a mineralocorticoid hormone produced by the adrenal glands. This leads to hypertension, hypokalemia, and metabolic alkalosis. The best test for diagnosing Conn syndrome should accurately measure aldosterone levels and differentiate it from other causes of hypertension.

**Why the Correct Answer is Right**
The best test for Conn syndrome is the **aldosterone-to-renin ratio (ARR)**. This ratio is calculated by measuring the levels of aldosterone and renin in the blood. In Conn syndrome, the aldosterone level is high and the renin level is low, resulting in a high ARR. This is because the excessive aldosterone production is not suppressed by the low renin levels, leading to a characteristic imbalance. The ARR is a sensitive and specific test for diagnosing Conn syndrome, especially when compared to other tests like the aldosterone suppression test.

**Why Each Wrong Option is Incorrect**
**Option A:** Plasma aldosterone concentration (PAC) alone is not sufficient for diagnosing Conn syndrome, as it can be elevated in other conditions as well. Therefore, it is not the best test for diagnosing Conn syndrome.
**Option B:** The aldosterone suppression test (AST) can be used to diagnose Conn syndrome, but it is less sensitive and more cumbersome than the aldosterone-to-renin ratio (ARR). It involves administering a low-dose dexamethasone to suppress aldosterone production, but it may not accurately differentiate Conn syndrome from other causes of hypertension.
**Option C:** The plasma renin activity (PRA) test measures the level of renin in the blood, but it is not sufficient for diagnosing Conn syndrome. In Conn syndrome, the renin level is low, but it can also be low in other conditions, making this test less specific.

**Clinical Pearl / High-Yield Fact**
The aldosterone-to-renin ratio (ARR) is a key test for diagnosing Conn syndrome, but it is essential to note that a high ARR can also be seen in patients with renovascular hypertension. Therefore, it is crucial to consider the clinical context and other diagnostic findings before making a diagnosis.

**Correct Answer: D. Aldosterone-to-renin ratio (ARR)**