A和C选项还有区别吗

What describes a swan neck deformity? 

A) Hyperextended metacarpophalangeal (MCP) and distal interphalangeal (DIP) joints, and  flexion deformity at the proximal interphalangeal (PIP) joint 

B) Synovitis at the ulnar styloid with resultant disruption of the ulnar collateral ligament 

C) Hyperextension of the MCP and DIP joints with flexion of the PIP joint 

D) MCP and PIP joint hyperextension with flexion deformity at the DIP joint 

100.D) Swan neck deformity is characteristic of rheumatoid arthritis. The deformity may start at the MCP , PIP , or DIP joint. If the flexor tendon at the MCP joint tightens, this may result in hyperextension at the PIP joint. Alternatively, if the PIP volar capsule becomes lax secondary to tenosynovitis, the PIP joint will hyperextend causing swan necking of the remaining joints. More commonly, however, stretching or disruption of the distal extensor mechanism results in a mallet finger deformity, which leads to eventual PIP hyperextension. 

A和C选项除了A当中有个deformity，A和C选项还有区别吗？

distal extensor mechanism是啥意思？