**Core Concept:**
Parotid gland calculi, also known as salivary calculi or sialoliths, are solid obstructions within the salivary ducts or gland parenchyma. They occur due to the precipitation and aggregation of salivary constituents like calcium salts, proteins, and salivary crystals, leading to duct obstruction and subsequent inflammation and infection.

**Why the Correct Answer is Right:**
The correct answer, **Option C: Ultrasonography (USG)**, is a non-invasive, radiation-free imaging modality used to diagnose and characterize salivary calculi. It provides detailed images of the salivary glands and ducts, allowing visualization of calculi size, location, and morphology. Ultrasonography can differentiate between calculi within the duct (intraductal) and those within the parenchyma (intraglandular), which is crucial for treatment planning.

**Why Each Wrong Option is Incorrect:**

**Option A: X-ray:** This imaging modality is usually not the first choice for diagnosing salivary calculi due to its limited spatial resolution and poor visualization of the intraductal calculi.

**Option B: Computed tomography (CT):** Although CT provides high spatial resolution, it exposes patients to ionizing radiation, which is undesirable in this context. Moreover, it lacks the dynamic assessment of the salivary glands and ducts that ultrasonography offers.

**Option D: Magnetic resonance imaging (MRI):** MRI is not a preferred choice due to its high cost, long examination time, and limited availability in some settings. Additionally, it is not as effective as ultrasonography in visualizing the intraductal calculi and assessing the surrounding salivary gland and duct anatomy.

**Clinical Pearl:**

Clinically, salivary calculi can cause symptoms like pain, swelling, and recurrent infections. Early diagnosis and treatment are crucial to prevent complications like sialadenitis, sialodochitis, and salivary gland dysfunction. Ultrasonography is a reliable, cost-effective, and radiation-free imaging modality for diagnosing salivary calculi, making it the preferred choice among the options provided.

**Correct Answer: Ultrasonography (USG)**

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Correct Answer: Ultrasonography (USG)

Ultrasonography (USG) is the preferred imaging modality for diagnosing salivary calculi due to its non-invasive nature and its ability to visualize the intraductal calculi, assess the surrounding salivary gland and duct anatomy, and differentiate between intraductal and intraglandular calculi. Other options like X-ray, computed tomography (CT), and magnetic resonance imaging (MRI) have limitations in terms of spatial resolution, radiation exposure, cost, and time consumption, making ultrasonography the superior choice for diagnosing salivary calculi.