The OK sign (thumb and index finger forming an O) tests which nerve function?

• A. Median nerve
• B. Ulnar nerve
• C. Radial nerve
• D. Axillary nerve

Answer: (A)

This test taps the motor function of the median nerve, specifically its anterior interosseous branch. The OK sign relies on a precise pinch between the thumb and index finger, which requires flexion at the thumb’s tip (flexor pollicis longus) and flexion at the index finger’s tip (flexor digitorum profundus to the index finger). Both of these muscles are innervated by the median nerve via the anterior interosseous nerve. If that pathway is impaired, these two flexors can’t contract properly, so the tips can’t meet to form a circle and the pinch loses its precision.

The other nerves don’t govern this exact pinch: the ulnar nerve mainly handles intrinsic hand muscles and finger adduction/abduction; the radial nerve controls wrist and finger extension; the axillary nerve supplies the deltoid and shoulder region. Thus the specific pinch-test deficit points to median nerve (AIN) function.