The ‘Terry Thomas sign’ (or David Letterman sign) is a classic radiographic finding seen on an anteroposterior (AP) clenched-fist view of the wrist. It specifically indicates an abnormal widening (>3 mm) caused by a complete rupture of which critical intrinsic intercarpal ligament?

Explanation:
The correct option is d. The scapholunate (SL) interosseous ligament is the primary stabilizer between the scaphoid and lunate. A complete tear (SL dissociation) results in a massive gap between these two bones on an AP radiograph, colloquially named after the comedian Terry Thomas (who had a prominent gap between his front teeth). If unrecognized, this ligamentous failure reliably progresses to Scapholunate Advanced Collapse (SLAC) wrist, a predictable pattern of devastating carpal osteoarthritis. Reference: Watson HK, Ballet FL. The SLAC wrist: scapholunate advanced collapse pattern of osteoarthritis. J Hand Surg Am. Essential orthopaedic mcq.