**Core Concept**
Ainhum is a rare condition characterized by a progressive constriction of the distal phalanx of the toe, often resulting from chronic inflammation and fibrosis. This constriction can lead to ischemia and eventual gangrene of the affected toe.

**Why the Correct Answer is Right**
The constriction in Ainhum typically develops at the interphalangeal joint of the toe due to the accumulation of fibrotic tissue and scarring. This process is thought to be triggered by chronic irritation or trauma to the toe, leading to a cascade of inflammatory responses and subsequent fibrosis. The interphalangeal joint is a common site for Ainhum due to its anatomical susceptibility to compression and friction.

**Why Each Wrong Option is Incorrect**
**Option A:** The constriction in Ainhum does not typically develop at the metatarsophalangeal joint.
**Option B:** The constriction in Ainhum is not usually caused by a bony deformity.
**Option C:** The constriction in Ainhum does not typically develop at the base of the toe.

**Clinical Pearl / High-Yield Fact**
Ainhum is often referred to as "dactylolysis spontanea" and is more commonly seen in individuals with a history of tight footwear or occupations that involve prolonged standing or walking.

**Correct Answer:** C.