A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 37-year-old man is brought to the emergency department after being attacked with a knife. Physical examination shows a 4-cm laceration in the midline of the right forearm. An MRI of the right arm shows damage to a nerve that runs between the superficial and deep flexor digitorum muscles. Loss of sensation over which of the following areas is most likely in this patient?
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A
Lateral aspect of the forearmIncorrect. The lateral forearm is innervated by the lateral antebrachial cutaneous nerve, a continuation of the musculocutaneous nerve — not affected by injury between FDS and FDP.
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B
Fingertip of the index fingerCorrect. the median nerve provides cutaneous sensation to the palmar and distal dorsal aspects of the index finger, including the fingertip.
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C
Medial aspect of the forearmIncorrect. The medial forearm is innervated by the medial antebrachial cutaneous nerve, a branch of the medial cord of the brachial plexus — not the median nerve.
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D
Dorsum of the thumbIncorrect. The dorsum of the thumb is in the territory of the superficial branch of the radial nerve, not the median nerve.
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E
Hypothenar eminenceIncorrect. Hypothenar skin (over the medial palm) is innervated by the superficial branch of the ulnar nerve, which travels through Guyon canal — not between FDS and FDP with the median nerve.
↑ Tap an answer to reveal the reasoning
Answer: B. An MRI of a forearm laceration showing damage to a nerve that runs BETWEEN the superficial and deep flexor digitorum muscles localizes the injury to the median nerve in the forearm — specifically the anterior interosseous portion, which lies between flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). The median nerve provides cutaneous sensation to the palmar surface of the lateral 3.5 digits (thumb, index, middle, and lateral half of ring) and the dorsal tips of those same fingers, including the dorsal aspect of the distal index fingertip.
Among the choices, only the fingertip of the index finger is a median-nerve territory. Sensory loss at the index fingertip is the classic median-nerve sensory deficit. The lateral forearm is supplied by the lateral antebrachial cutaneous nerve (musculocutaneous branch). The medial forearm is supplied by the medial antebrachial cutaneous nerve (medial cord). The dorsum of the thumb is the radial nerve territory (snuffbox/dorsum).
**Why each option:**
**A.** The lateral forearm is innervated by the lateral antebrachial cutaneous nerve, a continuation of the musculocutaneous nerve — not affected by injury between FDS and FDP.
**B.** Correct — the median nerve provides cutaneous sensation to the palmar and distal dorsal aspects of the index finger, including the fingertip.
**C.** The medial forearm is innervated by the medial antebrachial cutaneous nerve, a branch of the medial cord of the brachial plexus — not the median nerve.
**D.** The dorsum of the thumb is in the territory of the superficial branch of the radial nerve, not the median nerve.