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Which of the following muscles is still functioning in this patient?

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asked Apr 10, 2017 in Anatomy by Rajesh Bahara
An otherwise healthy infant boy born via vaginal delivery has an abnormal appearance of his right upper limb. His arm hangs by his side, pronated and medially rotated. Which of the following muscles is still functioning in this patient?

(A) Biceps
(B) Deltoid
(C) Infraspinatus
(D) Supraspinatus
(E) Triceps

1 Answer

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Best answer
(E) Triceps.
This infant has Erb- Duchenne palsy, a condition seen following trauma during delivery, due to traction/tear of the C5-C6 roots in the brachial plexus (superior trunk). This condition results in paralysis of abductors, lateral rotators, and biceps. Of the muscles listed above, only the triceps is not involved in these actions; it functions to extend the elbow and adduct the arm. It is innervated by the radial nerve, which is supplied by C6- C8.

Answer A is incorrect. The biceps muscle functions to supinate and flex the arm. It is innervated by the musculocutaneous nerve, which comes from the superior trunk of the brachial plexus.

Answer B is incorrect. The deltoid is supplied by the axillary nerve, which comes from the superior trunk of the brachial plexus. It functions to abduct the arm.

Answer C is incorrect. The infraspinatus muscle helps laterally rotate the arm and is supplied by the suprascapular nerve from the superior trunk of the brachial plexus.

Answer D is incorrect. The supraspinatus muscle helps abduct the arm and is supplied by the suprascapular nerve from the superior trunk of the brachial plexus.
answered Apr 10, 2017 by Rajesh Bahara

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