This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
Butonniere deformity occur due to -a) Flexion of Proximal interphalangeal jointb) Flexion at Distal interphalangeal jointc) Extension at Distal interphalangeal jointd) Extension at Metarcarpophalangeal jointe) Flexion at Metarcarpophalangeal joint 
Select and explain. 
A. a, B. ac, C. ad, D. b
B: ac
Answer: B: ac 
Explanation: None