**Core Concept**
Resistant hypertension refers to a condition where blood pressure remains elevated despite the concurrent use of three antihypertensive agents of different classes, at optimal doses, for a minimum of 8 weeks. This definition highlights the complexity of managing blood pressure in patients with resistant hypertension, often necessitating a multidisciplinary approach.

**Why the Correct Answer is Right**
The diagnosis of resistant hypertension requires the concurrent use of three antihypertensive agents, including a diuretic, a calcium channel blocker, or an angiotensin-converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB), and a third agent of any class. This combination is essential to ensure that the patient has received adequate therapy for their hypertension. The use of a diuretic, in particular, is crucial in managing fluid overload and electrolyte imbalances that can contribute to resistant hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the concurrent use of three antihypertensive agents at optimal doses for a minimum of 8 weeks.
**Option B:** This option is incorrect because it defines resistant hypertension based solely on the use of two antihypertensive agents, which is insufficient to diagnose this condition.
**Option C:** This option is incorrect because it defines resistant hypertension based solely on the use of a single antihypertensive agent, which is not sufficient to diagnose this condition.

**Clinical Pearl / High-Yield Fact**
The diagnosis of resistant hypertension requires a thorough evaluation of the patient's medication regimen, lifestyle factors, and underlying medical conditions, as well as close monitoring of blood pressure and electrolyte levels.

**Correct Answer: C. Blood pressure remains elevated despite the concurrent use of three antihypertensive agents of different classes, at optimal doses, for a minimum of 8 weeks.**