A 23-year-old woman who receives a deep cut to her ring finger by a kitchen knife is unable to move the metacarpophalangeal joint. Which of the following pairs of nerves was damaged?
**Core Concept**
The metacarpophalangeal (MCP) joint is a complex hinge joint responsible for flexion and extension of the fingers. Its movement is primarily controlled by the extrinsic muscles of the hand, which are innervated by specific branches of the brachial plexus. In this scenario, damage to the nerves controlling these muscles would result in impaired MCP joint movement.
**Why the Correct Answer is Right**
The movement of the MCP joint is mainly controlled by the extensor digitorum, extensor indicis, and extensor digiti minimi muscles, which are innervated by the radial nerve. Additionally, the flexor digitorum profundus muscle, responsible for flexion of the MCP joint, is innervated by the ulnar nerve. Damage to both the radial and ulnar nerves would result in impaired extension and flexion of the MCP joint, respectively, leading to the observed inability to move the joint.
**Why Each Wrong Option is Incorrect**
**Option A:** The median nerve primarily controls the flexor digitorum profundus and superficialis muscles, which are responsible for flexion of the MCP joint. However, it does not control the extension of the joint. The ulnar nerve, on the other hand, also controls flexion of the MCP joint, making the median and ulnar nerve combination incorrect.
**Option B:** The radial nerve primarily controls the extensor muscles of the wrist and fingers, whereas the median nerve controls the flexor muscles of the MCP joint. Damage to the radial and median nerves would result in impaired extension and flexion of the wrist and fingers, respectively, but not specifically the MCP joint.
**Option C:** The musculocutaneous nerve primarily controls the biceps brachii and brachialis muscles, which are responsible for flexion of the elbow joint, not the MCP joint. The ulnar nerve controls flexion of the MCP joint, making this combination incorrect.
**Clinical Pearl / High-Yield Fact**
The radial and ulnar nerves are often referred to as the "motor nerves of the hand." Damage to these nerves can result in significant impairment of hand function, including loss of extension and flexion of the MCP joint.
**✓ Correct Answer: D. Ulnar and radial. The ulnar and radial nerves were damaged.**