**Core Concept**
Doppler ultrasonography is a non-invasive imaging modality that uses the Doppler effect to detect and measure blood flow velocities in the carotid arteries. This technique is particularly useful for identifying stenosis or occlusion in asymptomatic patients, allowing for early detection and management of potential stroke risk.

**Why the Correct Answer is Right**
Doppler ultrasonography is the investigation of choice for asymptomatic patients presenting with a carotid bruit due to its non-invasive nature, low cost, and high sensitivity for detecting carotid stenosis. The Doppler effect allows for the measurement of blood flow velocities, which can identify areas of turbulent flow indicative of stenosis or occlusion. This modality is also useful for assessing the severity of stenosis and monitoring disease progression over time.

**Why Each Wrong Option is Incorrect**
**Option B:** Internal carotid angiography is an invasive procedure that carries a higher risk of complications, making it less suitable for asymptomatic patients. While it provides detailed images of the carotid arteries, its use is generally reserved for patients with symptoms or those who have failed non-invasive testing.

**Option C:** Aortic arch angiography is not the most appropriate choice for evaluating carotid artery disease, as it focuses on the aortic arch rather than the carotid arteries. This modality may be used in conjunction with carotid angiography but is not the primary investigation for carotid disease.

**Option D:** Spiral CT angiography is a sensitive modality for detecting carotid stenosis but is more expensive and exposes patients to radiation. It is typically reserved for patients with high-risk features or those who have failed non-invasive testing.

**Clinical Pearl / High-Yield Fact**
In asymptomatic patients with a carotid bruit, Doppler ultrasonography is the first-line investigation due to its non-invasive nature and high sensitivity for detecting carotid stenosis. This approach allows for early detection and management of potential stroke risk, reducing the need for more invasive procedures.

**✓ Correct Answer: A. Doppler ultrasonography**