In a complete, high-grade rupture of the ulnar collateral ligament (UCL) of the thumb metacarpophalangeal joint, a ‘Stener lesion’ occurs when the torn, retracted proximal end of the ligament becomes mechanically displaced and trapped superficial to which precise anatomical structure, absolutely mandating operative repair?

Explanation:
The correct option is c. A Stener lesion represents a true mechanical block to healing. When the UCL completely avulses from its distal insertion on the proximal phalanx, it can retract proximally and flip completely over the proximal edge of the adductor pollicis aponeurosis. The aponeurosis now sits directly between the torn ligament ends, making natural biological healing impossible regardless of how long the thumb is immobilized in a cast. Thus, identifying a Stener lesion (often via MRI or ultrasound) is an absolute indication for surgical intervention. Reference: Stener B. Displacement of the ruptured ulnar collateral ligament of the meta-carpophalangeal joint of the thumb. J Bone Joint Surg Br. Core Ortho mcq.