A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 36-year-old woman comes to the physician because of new onset limping. For the past 2 weeks, she has had a tendency to trip over her left foot unless she lifts her left leg higher while walking. She has not had any trauma to the leg. She works as a flight attendant and wears compression stockings to work. Her vital signs are within normal limits. Physical examination shows weakness of left foot dorsiflexion against minimal resistance. There is reduced sensation to light touch over the dorsum of the left foot, including the web space between the 1st and 2nd digit. Further evaluation is most likely to show which of the following?
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A
Decreased ankle jerk reflexIncorrect. Ankle jerk is an S1-mediated reflex (gastrocnemius/soleus via tibial nerve). A common peroneal palsy does not involve S1 and the ankle reflex remains intact.
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B
Normal foot eversionIncorrect. Eversion is performed by fibularis longus and brevis, both innervated by the superficial peroneal nerve. With a common peroneal lesion, eversion is WEAK, not normal.
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C
Normal foot inversionCorrect. Inversion is performed by tibialis posterior (tibial nerve) with assistance from tibialis anterior. Because the tibial nerve is spared in an isolated peroneal palsy, inversion remains normal — this is the key feature distinguishing peroneal palsy from L5 radiculopathy.
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D
Weak hip flexionIncorrect. Hip flexion is mediated by iliopsoas (L1–L3, femoral and lumbar plexus). It is unaffected by a common peroneal nerve lesion at the fibular head.
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E
Weak plantar flexionIncorrect. Plantar flexion is mediated by the tibial nerve (gastrocnemius/soleus) and is preserved in an isolated common peroneal nerve palsy; weakness of plantar flexion would suggest a more proximal sciatic or tibial lesion.
↑ Tap an answer to reveal the reasoning
Answer: C. The clinical picture is classic for a common peroneal (fibular) nerve palsy: foot drop (compensatory steppage gait, weak dorsiflexion), and sensory loss over the dorsum of the foot, especially the first web space (which is the signature sensory territory of the deep peroneal nerve). The common peroneal nerve is vulnerable as it wraps around the fibular neck, where prolonged compression (tight compression stockings, crossed legs, leg casts, sustained squatting) can injure it.
The nerve splits into the superficial peroneal branch (which innervates foot evertors — fibularis longus and brevis) and the deep peroneal branch (which innervates dorsiflexors — tibialis anterior, extensor hallucis longus, and extensor digitorum longus). Therefore peroneal palsy weakens BOTH dorsiflexion and eversion. Inversion (tibialis posterior, tibialis anterior) depends mostly on the tibial nerve and the deep peroneal contribution — in an isolated peroneal lesion, tibial-mediated inversion is preserved.
This distinguishes peroneal palsy from L5 radiculopathy (also weak dorsiflexion, but additionally weak inversion via tibialis posterior and weak hip abduction via gluteus medius) and from sciatic nerve injury (would also involve plantar flexion and tibial-distribution sensory loss). The ankle jerk is mediated by S1 and is preserved here.
**Why each option:**
**A.** Ankle jerk is an S1-mediated reflex (gastrocnemius/soleus via tibial nerve). A common peroneal palsy does not involve S1 and the ankle reflex remains intact.
**B.** Eversion is performed by fibularis longus and brevis, both innervated by the superficial peroneal nerve. With a common peroneal lesion, eversion is WEAK, not normal.
**C.** Inversion is performed by tibialis posterior (tibial nerve) with assistance from tibialis anterior. Because the tibial nerve is spared in an isolated peroneal palsy, inversion remains normal — this is the key feature distinguishing peroneal palsy from L5 radiculopathy.
**D.** Hip flexion is mediated by iliopsoas (L1–L3, femoral and lumbar plexus). It is unaffected by a common peroneal nerve lesion at the fibular head.