In the hand anatomy, Structure I corresponds to which joint in the provided mappings?

• A. Second proximal interphalangeal joint
• B. First carpometacarpal joint
• C. Second distal interphalangeal joint
• D. Second metacarpophalangeal joint

Answer: (A)

Structure I is at the joint between the proximal and middle phalanges of the index finger, which is the proximal interphalangeal joint. This hinge joint allows flexion and extension of the finger’s middle segment. The other options describe different joints: the base-of-thumb carpometacarpal joint, the interphalangeal joint between middle and distal phalanges, and the metacarpophalangeal joint between the index finger’s proximal phalanx and the second metacarpal. The position of Structure I fits the proximal interphalangeal joint of the index finger.