**Core Concept**
A fistula-in-ano is an abnormal connection between the epithelialized tract of an anal gland or the rectum and the skin near the anus, often resulting from a chronic infection or inflammatory process. Accurate diagnosis and classification of the fistula are crucial for guiding treatment.

**Why the Correct Answer is Right**
The ideal investigation for fistula-in-ano is a **surgical investigation**, specifically a **seton placement** or **mucosal advancement flap**, but more commonly an **end-to-end fistulography** or **MRI**. These methods help to identify the location, extent, and complexity of the fistula. End-to-end fistulography involves injecting a contrast agent through the internal opening of the fistula and taking X-rays to demonstrate the tract. MRI provides detailed images of the fistula, allowing for accurate classification and planning of surgical intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** Endoscopy is not ideal for fistula-in-ano as it may not visualize the entire fistulous tract, and the scope may not be able to reach the internal opening of the fistula.
**Option B:** CT scan is not typically used for fistula-in-ano as it may not provide sufficient detail for accurate diagnosis and may expose the patient to unnecessary radiation.
**Option C:** Colonoscopy is not relevant for fistula-in-ano as it is primarily used for examining the colon and rectum for polyps, cancer, and other disorders.

**Clinical Pearl / High-Yield Fact**
The Parks classification system is a useful tool for categorizing fistulas-in-ano based on their complexity and location, helping surgeons to plan the most effective treatment approach.

**Correct Answer:** MRI.