## **Core Concept**
Renovascular hypertension is a condition caused by the narrowing of the arteries that supply blood to the kidneys, leading to decreased renal perfusion. This results in the activation of the renin-angiotensin-aldosterone system (RAAS), causing increased blood pressure. The diagnosis often involves imaging studies to visualize the renal arteries and assess blood flow.

## **Why the Correct Answer is Right**
The correct answer, **C. Angiography (specifically, Digital Subtraction Angiography or DSA)**, is considered the gold standard for diagnosing renovascular hypertension, especially when there is a high suspicion of renal artery stenosis. Angiography provides direct visualization of the renal arteries, allowing for precise measurement of stenosis and assessment of the arterial lumen. This is particularly important in patients with asymmetrical kidneys on ultrasound, as it suggests possible differential renal perfusion due to renovascular disease.

## **Why Each Wrong Option is Incorrect**
- **Option A: Ultrasound Doppler** is a useful initial screening tool for assessing renal artery stenosis, especially for detecting significant stenosis and evaluating kidney size and echotexture. However, it may not provide the detailed visualization needed for planning interventions and is operator-dependent.
- **Option B: CT Angiography** is a highly sensitive and specific non-invasive test for detecting renal artery stenosis. While it is often used as a first-line imaging test due to its wide availability and speed, it may not always provide the same level of detail as DSA, particularly in cases of severe calcification or when intervention is planned.
- **Option D: MRI (MRA)** is another non-invasive imaging modality that can be used to evaluate the renal arteries. It is particularly useful in patients with contraindications to CT contrast agents. However, like CT angiography, it may not always match the resolution and detail provided by DSA.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while non-invasive imaging modalities like Doppler ultrasound, CT angiography, and MRI/MRA are often used as initial tests for suspected renovascular hypertension, **DSA remains the gold standard** for definitive diagnosis and for guiding endovascular interventions, such as angioplasty and stenting.

## **Correct Answer: C. Angiography**