SCM Innervation — MFDS Part 1 MCQ

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ModerateAnatomy (Head & Neck)SCM InnervationMFDS Part 1

Following a lymph-node biopsy in the right posterior triangle of the neck, a patient has weakness when turning the head to the left against resistance and difficulty elevating the right shoulder. Which nerve has most likely been injured?

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Correct answer: A — Accessory nerve (cranial nerve XI)

The correct answer is A, the accessory nerve (cranial nerve XI). Its spinal component supplies the sternocleidomastoid and trapezius. The right sternocleidomastoid rotates the head to the left, while the right trapezius contributes to shoulder elevation; weakness of both therefore localises the lesion to the right accessory nerve, which is vulnerable during procedures in the posterior triangle. Proprioceptive and other sensory fibres reach these muscles through the cervical plexus. The facial nerve supplies muscles of facial expression, the hypoglossal nerve supplies tongue muscles, V3 supplies the muscles of mastication, and the glossopharyngeal nerve provides motor supply only to stylopharyngeus.

Reference: Bordoni B, Varacallo MA. Neuroanatomy, Cranial Nerve 11 (Accessory). StatPearls; updated 2025. https://pubmed.ncbi.nlm.nih.gov/29939544/