**Core Concept**
The question pertains to the classification of anal fissures based on their location and potential for self-cleansing. Anal fissures are tears in the anal mucosa, which can be classified into different types based on their location and the presence of a sentinel pile.

**Why the Correct Answer is Right**
The correct answer is **C. Transverse fissure**. Transverse fissures are those that occur in the middle of the anal canal and are not self-cleansable because they are not associated with a sentinel pile, which is a small pile of skin that forms at the edge of the fissure. The presence of a sentinel pile in longitudinal fissures allows for better drainage and self-cleansing, whereas transverse fissures are more prone to infection and require surgical intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** **Vertical fissure** is actually a synonym for longitudinal fissure, which is self-cleansable due to the presence of a sentinel pile.
**Option B:** **Horizontal fissure** is another term for transverse fissure, which is not self-cleansable.
**Option D:** **Lateral fissure** is a type of fissure that occurs on the side of the anal canal and can be self-cleansable, depending on its location.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between longitudinal and transverse fissures, as the latter often requires surgical intervention due to its non-self-cleansable nature.

**Correct Answer: C. Transverse fissure**