**Core Concept**
Mineralocorticoid excess can lead to hypertension and hypokalemia due to the inappropriate activation of mineralocorticoid receptors by cortisol or other corticosteroids. The evaluation of mineralocorticoid excess involves assessing the ratio of cortisol to corticosteroid-binding globulin (CBG) in saliva or plasma, but a more practical screening test is needed.

**Why the Correct Answer is Right**
The correct screening test for evaluating mineralocorticoid excess is the **Renin-Aldosterone Ratio (RAR)**. This test measures the plasma levels of renin and aldosterone, which are key factors in the renin-angiotensin-aldosterone system (RAAS). An elevated aldosterone level, particularly in the presence of suppressed renin levels, suggests mineralocorticoid excess. This ratio is a useful screening tool as it can differentiate between primary aldosteronism and other causes of hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** Plasma cortisol levels are not a reliable screening test for mineralocorticoid excess as they can be elevated due to various causes, including Cushing's syndrome.
**Option B:** The aldosterone-to-renin ratio is the correct test, not just the renin level.
**Option C:** The plasma renin activity is a component of the Renin-Aldosterone Ratio but is not a standalone screening test.

**Clinical Pearl / High-Yield Fact**
Remember that primary aldosteronism is often associated with an elevated aldosterone level and suppressed renin level, which can lead to hypertension and hypokalemia.

**Correct Answer:** C.