**Core Concept**
Renovascular hypertension is caused by stenosis of renal arteries, leading to reduced renal perfusion and activation of the renin-angiotensin-aldosterone system. The most specific and sensitive test to detect such stenosis must accurately visualize the renal arteries with high spatial resolution and detectable vessel narrowing.

**Why the Correct Answer is Right**
CT Angiography (CTA) provides high-resolution, three-dimensional imaging of renal arteries and can detect stenosis with 90–95% sensitivity and specificity. It directly visualizes the arterial lumen, allowing precise measurement of stenosis severity, and is non-invasive with rapid turnaround. CTA is superior to Doppler ultrasound due to better image quality and operator dependence, and more reliable than radionucleotide scans which are less sensitive and prone to false positives.

**Why Each Wrong Option is Incorrect**
Option A: HRCT (High-Resolution CT) detects calcifications and structural changes but lacks vascular detail and cannot reliably assess renal artery stenosis.
Option C: Captopril-enhanced radionucleotide scan is less sensitive (50–70%) and may miss small stenoses; it is not specific due to confounding factors like glomerular filtration rate.
Option D: Doppler ultrasound is operator-dependent, has limited sensitivity in obese patients or when vessels are not optimally accessible, and lacks reproducibility compared to CTA.

**Clinical Pearl / High-Yield Fact**
CT Angiography is the gold standard for diagnosing renal artery stenosis due to its balance of sensitivity, specificity, accuracy, and accessibility—making it the first-line imaging modality in suspected renovascular hypertension.

✓ Correct Answer: B. CT Angiography