A 32-year-old man presents with wrist drop following a fracture of the mid-shaft humerus. On examination, he has weakness of wrist extension but preserved finger extension. Which clinical feature best distinguishes a lesion at the spiral groove from a lesion distal to the posterior interosseous nerve (PIN) origin?

Radial Nerve Anatomy and Lesion Localization

Course of the Radial Nerve

The radial nerve follows a critical anatomical path through the upper limb:

Key Anatomical Distinction

Why Sensation Is the Discriminator

A lesion distal to the PIN origin (e.g., posterior interosseous nerve syndrome at the radial tunnel) affects only the motor PIN; the superficial radial nerve is already separate and unaffected → no sensory loss over the dorsal first web space.

Why Option C Is Incorrect as a Discriminator

Option C ("weakness of finger extension with preserved wrist extension") describes the classic finding of a proximal PIN lesion (e.g., radial tunnel syndrome), not a feature that distinguishes spiral groove from distal-to-PIN lesions. Furthermore, the stem itself already states preserved finger extension, making Option C circular and non-discriminating. Both spiral groove and distal PIN lesions can produce finger extension loss; the sensory status is the reliable localizing feature.

Mnemonic

"Spiral = Sensory + Motor lost; PIN-only = Motor lost, Sensation Spared"

Reference: Gray's Anatomy for Students, 4th ed.; Aids to the Examination of the Peripheral Nervous System (Medical Research Council); Netter's Clinical Anatomy.

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