The classic, functionally limiting ‘Swan neck’ deformity of a digit in rheumatoid arthritis or chronic trauma is fundamentally characterized by which specific, sequential combination of joint malalignments?

Explanation:
The correct option is a. The Swan neck deformity is the exact mechanical opposite of the Boutonniere deformity. It begins with the primary attenuation or rupture of the volar plate at the PIP joint, allowing the PIP to uncontrollably hyperextend. This hyperextension puts immense tension on the FDP tendon and alters the extensor mechanism dynamics, mechanically driving the DIP joint into a reciprocal resting flexed posture. In rheumatoid patients, this is highly disabling as it prevents making a full fist. Reference: Campbell's Operative Orthopaedics. Classic Ortho mcq pathology.