Which nerve innervates the first dorsal interosseous muscle?

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

Answer: (A)

The key idea is the innervation pattern of the intrinsic hand muscles. The dorsal interossei, including the first one, are responsible for abducting the fingers away from the midline, with the first dorsal interosseous specifically acting to abduct the index finger. These interossei are supplied by the deep branch of the ulnar nerve, which provides motor innervation to most intrinsic hand muscles (except the thenar group and the lateral two lumbricals, which receive median nerve innervation). The deep branch travels through the hand to reach the interossei and the adductor pollicis. So, the first dorsal interosseous muscle is innervated by the ulnar nerve.