**Core Concept**
Carotid artery stenosis screening involves identifying and measuring the degree of narrowing in the carotid arteries, which supply blood to the brain. Accurate assessment is crucial to prevent stroke. Non-invasive imaging techniques are preferred for screening due to their safety and efficacy.

**Why the Correct Answer is Right**
Doppler ultrasound is widely used for in-vivo screening of carotid artery stenosis. It employs the Doppler effect to measure blood flow velocity and direction, allowing for the detection of stenosis and assessment of its severity. This technique is non-invasive, relatively inexpensive, and readily available. Doppler ultrasound can also measure the peak systolic velocity (PSV) and end-diastolic velocity (EDV) across the stenotic segment, which helps in grading the stenosis.

**Why Each Wrong Option is Incorrect**
**Option A:** USG (Ultrasound) is a broader term that encompasses Doppler ultrasound, but it does not specifically refer to the Doppler technique used in carotid artery stenosis screening.
**Option B:** CT (Computed Tomography) scans can be used to evaluate carotid artery stenosis, but they involve radiation and contrast agents, making them less ideal for screening purposes. CT angiography is more suitable for evaluating the extent of stenosis and planning interventions.
**Option C:** MRI (Magnetic Resonance Imaging) is not typically used for carotid artery stenosis screening due to its higher cost and limited availability compared to Doppler ultrasound.

**Clinical Pearl / High-Yield Fact**
When using Doppler ultrasound for carotid artery stenosis screening, it's essential to measure the PSV and EDV across the stenotic segment to accurately grade the stenosis. A PSV of > 4 m/s is generally considered indicative of significant stenosis.

**Correct Answer:**
✓ Correct Answer: D. Doppler