**Question:** Investigation of choice for parotid gland calculi

**Core Concept:** Parotid gland calculi are hard, non-tender, and movable masses within the major salivary glands, most commonly occurring in the parotid gland. They are caused by the precipitation of calcium salts from supersaturated saliva.

**Why the Correct Answer is Right:**

The correct answer, **Option D:** Ultrasonography (USG), is the preferred investigation for parotid gland calculi due to several reasons:

1. **Non-invasive and safe:** Ultrasonography is a safe, non-invasive imaging modality that can visualize the parotid gland and its surrounding structures without exposing the patient to radiation or contrast agents.

2. **High sensitivity and specificity:** Ultrasonography has high sensitivity (ability to detect true cases) and specificity (ability to exclude false positives), making it a reliable tool in diagnosing parotid gland calculi.

3. **Visualization of calculi size, number, and location:** Ultrasonography can accurately measure the size, number, and location of calculi, which is important for treatment planning and prognosis.

4. **Comparison with adjacent structures:** Ultrasonography can provide anatomic details of the parotid gland, facial nerve, and other nearby structures, helping to rule out other causes of parotid swelling.

5. **Versatility in assessing other salivary glands:** Ultrasonography can also assess other salivary glands if required, making it a comprehensive imaging modality for salivary gland diseases.

**Why Each Wrong Option is Incorrect:**

**Option A (CT Scan):** Computed tomography (CT) scan is more commonly used for skeletal or soft tissue abnormalities but lacks the resolution and real-time imaging capabilities of ultrasonography, making it less reliable for diagnosing parotid gland calculi.

**Option B (MRI):** Magnetic resonance imaging (MRI) is valuable in assessing soft tissue abnormalities, but it is more expensive, time-consuming, and lacks the real-time imaging of ultrasonography. MRI is also contraindicated in patients with certain implantable devices, pacemakers, or claustrophobia.

**Option C (Blood tests):** Blood tests, such as serum calcium or lithium levels, are helpful in evaluating systemic causes of parotid gland calculi but are not suitable for diagnosing the condition itself.

**Option E (Physical examination):** Physical examination can demonstrate the presence of a mass, but it lacks the ability to visualize the internal structures and calculi directly, making it less accurate for diagnosis.

**Clinical Pearls:**

1. **Parotid gland calculi are typically seen as hyperechoic (bright echoes) or hypoechoic (dark echoes) masses within the gland.**
2. **Ultrasonography can differentiate between simple parotid gland calculi and other causes of parotid gland swelling.**
3. **Ultrasonography is cost-effective, readily available, and safe for patients with certain contraind