**Core Concept**
A parotid fistula is an abnormal communication between the parotid gland and the skin, often resulting from trauma, surgery, or infection. The investigation of choice should ideally be non-invasive, sensitive, and specific for detecting the fistula.

**Why the Correct Answer is Right**
Sialography, specifically a parotid sialogram, is the investigation of choice for parotid fistula. It involves injecting a water-soluble contrast agent into the parotid duct, which then fills the parotid gland and any abnormal communications, allowing for visualization of the fistula on X-ray. This helps in diagnosing the exact location and extent of the fistula.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is not the investigation of choice for parotid fistula, although it may be used as an adjunct to sialography to assess the gland's anatomy and any associated pathology.
**Option B:** CT scan may be used to evaluate the extent of the fistula and surrounding tissues, but it is not the primary investigation of choice.
**Option C:** MRI may provide detailed images of the parotid gland and surrounding tissues, but it is not typically used for the primary investigation of parotid fistula due to its higher cost and limited availability.

**Clinical Pearl / High-Yield Fact**
Sialography is a useful tool in the diagnosis of parotid gland disorders, including fistulas, stones, and inflammation. It is essential to remember that sialography should be performed with a water-soluble contrast agent to avoid complications.

**Correct Answer: D. Sialography**