A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 25-year-old man was referred to a neurologist for right-hand weakness. He was involved in a motor vehicle accident 2 months ago in which his right hand was injured. On examination, his grip is weak, especially in fingers 2, 4, and 5 and he is unable to adduct these fingers. Which of the following groups of muscles is most likely affected?
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A
Palmar interossei musclesCorrect. Palmar interossei ADduct the fingers (PAD); their loss produces the inability to adduct fingers 2, 4, and 5 as described.
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B
Lumbrical musclesIncorrect. Lumbricals flex MCP and extend PIP/DIP joints; their dysfunction causes the ulnar/median 'claw' hand posture but does not produce isolated adduction weakness.
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C
Dorsal interossei musclesIncorrect. Dorsal interossei ABduct (DAB) the fingers; their loss would impair finger spreading, not finger adduction.
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D
Flexor digitorum profundusIncorrect. Flexor digitorum profundus flexes the DIP joints; weakness causes loss of distal finger flexion (e.g., pointing index finger sign), not loss of adduction.
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E
Adductor pollicisIncorrect. Adductor pollicis adducts the thumb; its loss produces a positive Froment sign (ulnar neuropathy) but does not affect adduction of fingers 2, 4, and 5, which is governed by the palmar interossei.
↑ Tap an answer to reveal the reasoning
Answer: A. Weakness of finger adduction (the ability to bring the fingers together) with preservation of finger flexion suggests injury to the ulnar nerve and specifically loss of the palmar interossei. The hand intrinsic mnemonic 'PAD-DAB' is helpful: Palmar interossei ADduct the fingers, Dorsal interossei ABduct the fingers. The palmar interossei (3 in number) are innervated by the deep branch of the ulnar nerve and act on digits 2, 4, and 5 — pulling them toward the middle finger (the axis of the hand). The middle finger (digit 3) has dorsal interossei on both sides but no palmar interossei because it IS the axis of ad/abduction.
Weak grip and inability to adduct fingers 2, 4, and 5 specifically implicates the palmar interossei. Ulnar nerve injury at the wrist or in the hand classically also impairs finger abduction (dorsal interossei), thumb adduction (adductor pollicis — positive Froment sign), and produces an 'ulnar claw' from loss of medial two lumbricals.
The lumbricals flex MCP and extend IP joints (their loss creates the claw posture); flexor digitorum profundus flexes DIP joints — distinct from the adduction deficit described.
**Why each option:**
**A.** Correct. Palmar interossei ADduct the fingers (PAD); their loss produces the inability to adduct fingers 2, 4, and 5 as described.
**B.** Lumbricals flex MCP and extend PIP/DIP joints; their dysfunction causes the ulnar/median 'claw' hand posture but does not produce isolated adduction weakness.
**C.** Dorsal interossei ABduct (DAB) the fingers; their loss would impair finger spreading, not finger adduction.
**D.** Flexor digitorum profundus flexes the DIP joints; weakness causes loss of distal finger flexion (e.g., pointing index finger sign), not loss of adduction.