**Core Concept**
Carotid artery stenosis screening involves assessing the degree of narrowing in the carotid arteries, which supply blood to the brain. The goal is to identify individuals at risk of stroke or transient ischemic attack (TIA). Doppler ultrasound is a non-invasive, widely used method for evaluating carotid artery stenosis.

**Why the Correct Answer is Right**
Doppler ultrasound measures blood flow and detects changes in velocity and direction, allowing for the assessment of stenosis. It is particularly useful for identifying high-grade stenosis (>70%) and detecting emboli or plaque characteristics. The Doppler effect is based on the principle that the frequency of sound waves changes when they encounter moving objects, such as blood cells. This technology is often used in conjunction with B-mode ultrasound to visualize the carotid artery and assess plaque burden.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a distractor, as it is not a commonly used method for carotid artery stenosis screening.
**Option B:** While CT angiography is useful for evaluating carotid artery stenosis, it involves radiation and contrast media, making it less suitable for initial screening.
**Option C:** Magnetic Resonance Angiography (MRA) is another imaging modality for carotid artery stenosis, but it is typically more expensive and time-consuming than Doppler ultrasound.

**Clinical Pearl / High-Yield Fact**
When interpreting Doppler ultrasound results, it is essential to consider the peak systolic velocity (PSV) and end-diastolic velocity (EDV) values, as these can help identify high-grade stenosis.

**Correct Answer: C. Magnetic Resonance Angiography (MRA)**