Compression of which nerve would cause weakness of the flexor pollicis brevis, sensory disturbance of the thumb and index finger, and diminished grip strength?

• A. Median
• B. Ulnar
• C. Deep posterior interosseous
• D. Radial

Answer: (A)

Compression of the median nerve affects the thenar muscles through its recurrent branch, including the flexor pollicis brevis. When this muscle is weakened, the thumb cannot flex and oppose properly, which is a key part of a strong grip. At the same time, the median nerve supplies sensation to the thumb and index finger, so compression leads to sensory disturbance in those areas. Because grip strength relies in part on good thumb opposition and flexion, losing function of these median- nerve–innervated components reduces overall grip strength.

The other nerves would produce different patterns: the ulnar nerve mainly affects the little and ring fingers and the interossei, not the thumb and index; the deep posterior interosseous nerve is a motor branch to some forearm extensors, not the thumb flexors or hand sensation; the radial nerve mainly causes problems with wrist and finger extension and dorsal hand sensation.