**Core Concept**
The question is testing the student's understanding of the diagnostic approach to renal artery hypertension, which involves identifying the most sensitive and specific investigation for this condition. Renal artery hypertension is a type of secondary hypertension caused by narrowing of the renal arteries, leading to reduced blood flow to the kidneys.

**Why the Correct Answer is Right**
The correct answer is **Captopril Renography**. Captopril renography is a functional imaging test that assesses renal function and blood flow. It involves administering captopril, an angiotensin-converting enzyme (ACE) inhibitor, which blocks the conversion of angiotensin I to angiotensin II. This leads to vasodilation of the efferent arterioles in the kidneys, causing a decrease in glomerular filtration rate (GFR) in the affected kidney. The test is highly sensitive and specific for detecting renal artery stenosis, as it can identify the kidney with reduced blood flow.

**Why Each Wrong Option is Incorrect**
* **Option A:** Renal ultrasound is a useful tool for detecting renal artery stenosis, but it is not as sensitive or specific as captopril renography for diagnosing renal artery hypertension.
* **Option B:** Magnetic resonance angiography (MRA) is a non-invasive imaging test that can visualize the renal arteries, but it may not be as sensitive as captopril renography in detecting renal artery stenosis.
* **Option D:** Computed tomography angiography (CTA) is a highly sensitive and specific imaging test for detecting renal artery stenosis, but it involves radiation and contrast exposure, making it less desirable than captopril renography.

**Clinical Pearl / High-Yield Fact**
Captopril renography is a useful test for diagnosing renal artery hypertension because it can identify the kidney with reduced blood flow, which can help guide treatment decisions.

**Correct Answer:** C. Captopril Renography