**Core Concept**
The diagnostic method for submandibular sialolithiasis involves identifying a radiopaque stone within the submandibular gland or its duct. This condition is characterized by the formation of a calcified stone that obstructs the flow of saliva.

**Why the Correct Answer is Right**
Submandibular sialolithiasis is typically diagnosed using plain radiographs, such as a lateral neck X-ray or a panoramic radiograph. These imaging modalities are effective in detecting the radiopaque nature of the stone, which appears as a calcified structure. The stone's location within the gland or duct is also visualized, aiding in the determination of the optimal treatment approach.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is not typically used for diagnosing submandibular sialolithiasis, as it may not accurately detect radiopaque stones.
**Option B:** CT scans are not the first-line diagnostic tool for submandibular sialolithiasis, although they may be used in complex cases.
**Option C:** MRI is not commonly used for diagnosing submandibular sialolithiasis, as it is not sensitive to calcifications.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of submandibular sialolithiasis is the presence of a palpable mass or swelling in the submandibular region, which may be accompanied by pain and difficulty swallowing.

**Correct Answer: C. CT scans are not the first-line diagnostic tool for submandibular sialolithiasis, although they may be used in complex cases.**