What deformity is indicated by an inability to flex the proximal phalanx of the fourth and fifth fingers?

The correct answer is associated with a condition characterized by the inability to flex the proximal phalanx of the fourth and fifth fingers, which is indicative of a claw hand deformity. This deformity typically arises due to ulnar nerve palsy, where the muscles innervated by the ulnar nerve are weakened. As a result, the flexor digitorum profundus and interossei muscles that help in flexing these fingers are impaired.

In claw hand, the fingers may exhibit a hyperextension at the metacarpophalangeal joints while remaining flexed at the interphalangeal joints. This particular feature is a classic sign of this condition, making it the most suitable description for the inability to flex the proximal phalanx of the fourth and fifth fingers.

Understanding this condition is crucial for recognizing symptoms and implementing the appropriate treatment and rehabilitation strategies, especially for athletes or individuals involved in activities that may lead to ulnar nerve injury.