Okay, let's tackle this question about the investigation of choice for submandibular sialolithiasis. First, I need to recall what sialolithiasis is. It's the presence of stones in the salivary glands, usually the submandibular gland. The submandibular duct is long and narrow, which makes it a common site for stones. Now, the question is asking which investigation is preferred here.

I remember that for salivary gland stones, imaging is key. The options might include ultrasound, CT, MRI, or sialography. Let's think. Ultrasound is non-invasive and can detect stones, especially in the submandibular area. Sialography is when you inject contrast into the duct and take X-rays. But it's invasive and might not be the first choice. CT is good for stones but involves radiation. MRI is better for soft tissues but might not show stones as well. So, the investigation of choice is probably ultrasound because it's non-invasive and effective for this area.

Wait, but I've heard that in some cases, sialography is used for the submandibular gland. Hmm. Maybe it's a bit tricky. Let me check. Submandibular sialolithiasis: the duct is long and runs upward, so stones can be seen on plain X-rays. But some stones are radiolucent. Sialography was used in the past, but now ultrasound is preferred. So the answer is probably ultrasound. Let me confirm. Yes, ultrasound is the first-line imaging modality for submandibular sialolithiasis because it's non-invasive, readily available, and can detect both radiopaque and radiolucent stones. Sialography is less commonly used now. CT might be used if the stone is not visible on ultrasound. So the correct answer should be ultrasound, which is option B perhaps. But I need to make sure the options given in the question are there. Wait, the user didn't provide the options. But the correct answer is ultrasound. So, in the explanation, I need to explain why ultrasound is the best, and why the other options are not. Let me outline the sections as per the user's instructions.

Core Concept: The submandibular gland's anatomy and the imaging techniques for sialolithiasis.

Why correct answer is right: Ultrasound is non-invasive, detects stones in the duct, and is preferred.

Why others are wrong: Sialography is invasive, CT has radiation, MRI doesn't show stones well.

Clinical pearl: Remember that submandibular stones are best seen on ultrasound.

Okay, structuring all that into the required format.

**Core Concept**
Submandibular sialolithiasis involves salivary gland stones in the **Wharton’s duct**, which is long and narrow, increasing its susceptibility to stone formation. The investigation of choice must detect **radiopaque and radiolucent stones** without invasiveness.

**Why the Correct Answer is Right**
**Ultrasonography** is the preferred imaging modality for submandibular sialolithiasis. It is non-invasive, readily available, and highly sensitive for detecting both calcified and non-calcified stones in the submandibular gland. Ultrasound also evaluates gland

✓ Correct Answer: D. X-ray