Step 1 topics → Neurology and Special Senses → Common Mononeuropathies

Common Mononeuropathies

USMLE Step 1 trap: Confuses the nerve responsible for wrist drop with the median nerve. Wrist drop results from radial nerve injury (e.g., midshaft humeral fracture or Saturday night palsy), not carpal tunnel compression of the median nerve.

Mononeuropathies are focal injuries to single peripheral nerves, and USMLE Step 1 loves testing them through clinical vignettes where you have to reverse-engineer the nerve from the deficit. The core skill isn't memorizing nerve names — it's mapping a specific motor loss, sensory distribution, or provocative sign back to the exact nerve and injury location. The exam hits this from three main angles: carpal tunnel syndrome (with its classic exam signs and risk factors), radial versus ulnar nerve palsy (wrist drop vs. claw hand), and peroneal versus tibial nerve injury (foot drop vs. plantarflexion loss).

What makes this topic tricky is that students blur the boundaries between nerves and their clinical syndromes. Wrist drop and carpal tunnel are both 'hand problems,' so students conflate them — but they involve completely different nerves and mechanisms. Similarly, foot drop sounds like it should involve the nerve that controls the foot's main power, but it's actually the smaller peroneal nerve, not the tibial. The exam exploits exactly these intuition traps.

Another common pitfall is incomplete sensory mapping. Carpal tunnel (median nerve compression) does NOT affect all five fingers — it spares the little finger and the medial half of the ring finger, which are ulnar territory. USMLE Step 1 will describe numbness in specific fingers and expect you to name the nerve precisely. Build your mental map finger-by-finger, not just 'hand' versus 'not hand.'

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Common misconceptions

Common mistake
Wrong: Wrist drop from radial nerve palsy is caused by compression at the carpal tunnel.
Right: Wrist drop results from radial nerve injury (e.g., midshaft humeral fracture or Saturday night palsy), not carpal tunnel compression of the median nerve.
Wrist drop is a radial nerve problem, not a median nerve problem — these two nerves don't even run through the carpal tunnel together. The radial nerve wraps around the posterior aspect of the humerus (spiral groove), making it vulnerable to midshaft humeral fractures and prolonged compression from sleeping with your arm over a chair back ('Saturday night palsy'). Carpal tunnel compresses the median nerve, causing thenar weakness and sensory loss in the lateral 3.5 fingers — not wrist drop. Keep these syndromes completely separate in your head.
Common mistake
Wrong: Carpal tunnel syndrome affects all five fingers equally.
Right: Carpal tunnel syndrome (median nerve) causes sensory loss in the lateral 3.5 fingers (thumb, index, middle, and lateral half of ring), sparing the little finger.
The median nerve only supplies the lateral 3.5 fingers: thumb, index, middle, and the lateral (radial) half of the ring finger. The medial half of the ring finger and the entire little finger are ulnar nerve territory, which does not pass through the carpal tunnel. So when carpal tunnel syndrome is described, the little finger should be completely unaffected — if the question mentions little finger numbness, think ulnar nerve (Guyon's canal compression or cubital tunnel), not carpal tunnel.
Common mistake
Wrong: Foot drop is caused by tibial nerve injury.
Right: Foot drop (loss of dorsiflexion) is caused by common peroneal nerve injury; tibial nerve injury causes loss of plantarflexion and toe flexion.
Foot drop means you can't lift the foot up (lost dorsiflexion), and this is a peroneal nerve problem. The common peroneal nerve wraps around the fibular head — it's vulnerable to lateral knee trauma, tight casts, or prolonged squatting — and it controls dorsiflexion and eversion via the anterior and lateral compartments. The tibial nerve runs posteriorly and controls plantarflexion (pushing down) and toe flexion. If a patient can't 'step up' or drags their toe walking, that's peroneal; if they can't push off the ground, that's tibial.
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What the exam tests

  1. Given a patient with hand numbness, thenar wasting, positive Tinel and Phalen signs, or a known risk factor (pregnancy, hypothyroidism, rheumatoid arthritis, repetitive wrist use), identify carpal tunnel syndrome and know exactly which fingers are affected and which are spared.
  2. Distinguish radial nerve palsy (wrist drop, loss of finger/wrist extension, spared intrinsic hand muscles) from ulnar nerve palsy (claw hand with ring/little finger involvement, loss of hypothenar and interosseous function, sensory loss in medial 1.5 fingers) based on a clinical presentation.
  3. Differentiate common peroneal nerve injury (foot drop, loss of dorsiflexion and eversion, sensory loss over dorsum of foot) from tibial nerve injury (loss of plantarflexion and toe flexion, sensory loss over sole) given a mechanism of injury or clinical findings.

Can you avoid these mistakes?

A 32-year-old pregnant woman reports waking up at night with burning numbness in her thumb, index finger, and middle finger. Shaking her hand relieves it briefly. Which nerve is affected, what space is it compressed in, and which finger would NOT be numb?
A man is found unconscious in an alley and wakes unable to extend his wrist or fingers, with sensory loss over the dorsum of the hand and first web space. Where is the most likely site of nerve compression, and which nerve is injured?
A patient sustains a lateral knee injury in a car accident and now cannot dorsiflex or evert the foot. He walks with a high-stepping gait. Which nerve is damaged, where does it typically get injured, and how would his presentation differ if the tibial nerve were injured instead?
A patient has weakness of the intrinsic hand muscles (interossei, hypothenar), a 'claw' deformity of the ring and little fingers, and numbness over the medial 1.5 fingers. Is this a median or ulnar nerve lesion, and what is one common site of compression for this nerve?

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