This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Upper Extremity, Subject name: Anatomy
A 45-year-old man visits the outpatient clinic after a digit of his left hand was injured when a door was slammed on his hand. A superficial cut on his middle finger has been sutured, but functional deficits are observed in the finger: The proximal interphalangeal joint is pulled into constant flexion, whereas the distal interphalangeal joint is held in a position of hyperextension. What is the most likely diagnosis? 
Select and explain. 
A. Mallet finger, B. Boutonniere deformity, C. Dupuytren's contracture, D. Swan-neck deformity
B: Boutonniere deformity
Answer: B: Boutonniere deformity 
Explanation: In boutonniere deformity, the central portion of the extensor tendon expansion is torn over the PIP joint, allowing the tendon to move palmarward, causing the tendon to act as a flexor of the PIP joint. This causes the DIP joint to be hyperextended. The tear in the extensor tendon is said to resemble a buttonhole ("boutonniere" in French), and the head of the proximal phalanx may stick through the hole.