La d A 7-year-old boy was brought to the hospital with multiple fractures of the humerus secondary to a fall from height. On examination, there is difficulty in flexion of the elbow and supination of the forearm and associated loss of sensation over the lateral aspect of the forearm. Which is the nerve most likely to be injured?

Correct Answer: A. Musculocutaneous nerve

The musculocutaneous nerve (C5–C7) is the terminal sensory and motor branch of the lateral cord of the brachial plexus. It arises from the axilla, pierces the coracobrachialis muscle, and descends between the biceps and brachialis muscles—the two primary elbow flexors. After exiting the forearm, it continues as the lateral antebrachial cutaneous nerve, supplying sensation to the lateral aspect of the forearm. In this case, the clinical triad—loss of elbow flexion (biceps and brachialis), loss of forearm supination (biceps is the primary supinator), and loss of sensation over the lateral forearm—is pathognomonic for musculocutaneous nerve injury. Humeral fractures, especially proximal and mid-shaft fractures, commonly injure the musculocutaneous nerve as it courses through the anterior arm. The nerve's superficial course through the biceps makes it vulnerable to traction and direct trauma during fracture displacement. This is a high-yield anatomy correlation in pediatric trauma cases.

Why the other options are wrong

B. Radial nerve — The radial nerve (C5–C8, T1) innervates the triceps (elbow extension) and extensor muscles of the forearm and wrist. Radial nerve injury causes wrist drop and loss of sensation over the dorsal first web space, NOT loss of elbow flexion or supination. Although radial nerve injury is common in mid-shaft humeral fractures, the clinical presentation here—preserved elbow extension with lost flexion—excludes radial involvement. C. Median nerve — The median nerve (C5–T1) innervates pronator teres, flexor carpi radialis, and intrinsic hand muscles. Median nerve injury causes loss of pronation and wrist flexion, not supination loss. Sensory loss in median nerve injury occurs over the lateral palm and lateral three-and-a-half fingers, not the lateral forearm. The preserved pronation and absence of hand sensory loss rule out median nerve injury. D. Ulnar nerve — The ulnar nerve (C8–T1) innervates intrinsic hand muscles and flexor carpi ulnaris. Ulnar nerve injury causes claw hand deformity and loss of sensation over the medial one-and-a-half fingers and medial forearm. The clinical presentation—lateral forearm sensory loss and elbow flexion loss—is inconsistent with ulnar nerve injury, which does not affect elbow flexion or lateral forearm sensation.

High-Yield Facts

Mnemonics

MCN Triad Motor loss (elbow Flexion + forearm Supination) + Sensory loss (Lateral forearm) = MCN injury. Use when you see elbow flexion loss in upper limb trauma. Nerve-Sensory Zones (Upper Limb) Musculocutaneous → lateral forearm; Radial → dorsal first web space; Median → lateral palm; Ulnar → medial palm. Lateral forearm sensory loss = musculocutaneous.

NBE Trap

NBE may pair radial nerve injury with humeral fractures to trap students who know radial nerve is common in mid-shaft fractures but forget that radial injury causes wrist drop (extension loss), not elbow flexion loss. The question's emphasis on elbow flexion loss is the discriminator.

Clinical Pearl

In Indian pediatric trauma centers, musculocutaneous nerve injury from humeral fractures is often missed because clinicians focus on radial nerve examination. Always test elbow flexion (against resistance) and forearm supination in every upper limb fracture—a child who cannot flex the elbow against gravity has a musculocutaneous nerve injury until proven otherwise.

_Reference: Bailey & Love Ch. 38 (Upper Limb Nerve Injuries); Robbins Ch. 27 (Peripheral Nerve Pathology)_