**Core Concept:** Butonniere deformity is a clinical condition characterized by flexion deformity at the distal interphalangeal joint (DIPJ) and limited extension at the proximal interphalangeal joint (PIPJ). This leads to a characteristic "V" or "banana" shape of the affected finger.

**Why the Correct Answer is Right:** Butonniere deformity occurs due to limited extension at the DIPJ, which results in a flexion contracture. This can be caused by a combination of factors like trauma, infection, or inflammatory conditions affecting the PIPJ and DIPJ complex. The correct answer is option C, which focuses on the extension defect at the DIPJ.

**Why Each Wrong Option is Incorrect:**

A) Flexion at the PIPJ: This option is incorrect because Butonniere deformity is characterized by flexion at the DIPJ, not PIPJ.

B) Flexion at the DIPJ: Similar to option A, this incorrectly focuses on the wrong joint involved in the deformity.

D) Extension at the MCPJ: Butonniere deformity involves the DIPJ and PIPJ, not the metacarpophalangeal joint (MCPJ).

E) Flexion at the MTPJ: Butonniere deformity is related to the DIPJ and PIPJ, not the metatarsophalangeal joint (MTPJ).

**Clinical Pearl:** Butonniere deformity is a common clinical entity in the field of hand surgery, often resulting from trauma or infection. Early recognition and treatment are crucial to prevent progressive deformity and restore normal finger function.

**Correct Answer:** C) Flexion at the DIPJ.