**Core Concept**
The patient has resistant hypertension, which is defined as blood pressure that remains elevated despite the concurrent use of three antihypertensive agents of different classes, including a diuretic. This condition requires further evaluation to identify potential underlying causes.

**Why the Correct Answer is Right**
In this case, the patient has already been started on a thiazide diuretic, which is a common first-line treatment for hypertension. However, the patient's blood pressure remains elevated, suggesting that other factors may be contributing to his condition. The next helpful step in the diagnosis would be to check for secondary causes of hypertension, particularly those that can be treated or managed. One of the most common secondary causes of hypertension is primary aldosteronism, which can be diagnosed with a plasma aldosterone to renin ratio (ARR) test.

**Why Each Wrong Option is Incorrect**
**Option A:** Checking the patient's kidney function through serum creatinine and eGFR is not the next helpful step, as the patient's electrolytes are normal, and there is no indication of kidney disease.

**Option B:** Ordering an echocardiogram to assess cardiac function may provide some information but is not the most helpful next step in diagnosing resistant hypertension.

**Option C:** Prescribing a fourth antihypertensive agent without further evaluation may not be effective and could lead to adverse effects.

**Option D:** Ordering a 24-hour urine cortisol level to diagnose Cushing's syndrome is not the most appropriate next step, as this condition is a rare cause of hypertension.

**Clinical Pearl / High-Yield Fact**
Resistant hypertension is a common condition that requires a thorough evaluation to identify potential underlying causes. A systematic approach to diagnosis, including checking for secondary causes of hypertension, is essential to ensure effective management and treatment.

**Correct Answer:** C. Prescribing a fourth antihypertensive agent without further evaluation may not be effective and could lead to adverse effects.