Nerve conduction studies in a 40-year-old woman with distal numbness show markedly slowed conduction velocities (22 m/s) in motor nerves with relatively preserved amplitudes. The predominant pathological process is:

• A Axonal degeneration of large myelinated fibers
• B Loss of small unmyelinated C fibers carrying pain sensation
• C Failure of the Na-K pump reducing the safety factor for propagation
• D Segmental demyelination disrupting saltatory conduction ✓

Explanation

Conduction velocity depends critically on internodal myelin, which allows saltatory jump between nodes of Ranvier. Demyelination forces slow continuous conduction, slashing velocity while axonal continuity preserves amplitude until late. Pure axonal neuropathies produce reduced amplitudes with comparatively preserved velocities. The distractor B involves fibers too fine to be assessed on routine motor studies, and C is a metabolic concept irrelevant to measured velocity in this setting.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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