**Core Concept**
The diagnosis of renovascular hypertension (RVH) involves identifying stenosis or occlusion of the renal arteries, which can be caused by atherosclerosis, fibromuscular dysplasia, or other conditions. Accurate imaging is crucial for diagnosing RVH, as it can mimic other causes of secondary hypertension.

**Why the Correct Answer is Right**
The most sensitive and specific investigation for screening RVH is the **Angiography**. This involves injecting a contrast dye into a blood vessel and then taking X-ray images to visualize the renal arteries. Angiography can directly visualize the site and extent of stenosis or occlusion, making it the gold standard for diagnosing RVH. **Digital subtraction angiography (DSA)** is a type of angiography that provides high-resolution images and is particularly useful for identifying small lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** **Magnetic Resonance Angiography (MRA)** is less sensitive and specific than angiography for detecting RVH, particularly in patients with renal insufficiency. Although MRA is non-invasive and provides excellent visualization of the renal arteries, it can miss small lesions and is not as accurate as angiography.

**Option B:** **Computed Tomography Angiography (CTA)** is a sensitive but not specific imaging modality for RVH. While CTA can detect stenosis or occlusion of the renal arteries, it can also be affected by renal function and may not provide the same level of detail as angiography.

**Option C:** **Doppler Ultrasound** is a non-invasive imaging modality that can detect changes in blood flow through the renal arteries. However, it is not as sensitive or specific as angiography for detecting RVH, particularly in patients with small lesions or renal insufficiency.

**Option D:** **Renal vein renin sampling** is an invasive procedure that involves collecting blood samples from the renal veins to measure renin levels. While this test can help diagnose RVH, it is not a primary imaging modality and is typically reserved for patients with equivocal imaging results.

**Clinical Pearl / High-Yield Fact**
When diagnosing RVH, it's essential to remember that **MRA and CTA may be affected by renal function**, and **Doppler ultrasound may not detect small lesions**. Therefore, angiography remains the gold standard for diagnosing RVH.

**Correct Answer:** A. Angiography