Question

Which muscle is paralyzed if there is hyperextension of metacarpophalangeal joint and flexion of the interphalangeal joint?

• A. Extensor digitorum
• B. Interossei and lumbricals
• C. Adductor pollicis
• D. Pronator quadratus muscle

Answer 1

The Correct Option is B

Action-reversal angle. Picture the hand at rest: the lumbricals and interossei are the only muscles that simultaneously flex the MCP joints and extend the IP joints, balancing the pull of the long extensors and flexors. Knock them out and you read off the opposite of their action: the MCP joints fall into hyperextension (long extensors now unopposed) and the IP joints fall into flexion (long flexors now unopposed). That deformity is the claw hand, the hallmark of ulnar nerve palsy and worse in combined ulnar-median lesions. Run a quick contrast with the distractors: extensor digitorum paralysis gives wrist or finger drop, not clawing; adductor pollicis loss shows up as Froment sign at the thumb; and pronator quadratus is a deep forearm pronator with no role at the fingers. So the muscles paralysed are the interossei and lumbricals. Reference: BDC, 7e, Vol 1, p 163.