A 45-year-old man undergoes biopsy of a left supraclavicular lymph node. Postoperatively he has an elevated left hemidiaphragm on chest radiograph and breathlessness on lying flat. The injured nerve was lying directly on which structure?
- A Posterior surface of scalenus medius
- B Anterior surface of scalenus anterior ✓
- C Anterior surface of scalenus medius
- D Superior surface of the first rib between the scalene muscles
Correct answer: B. Anterior surface of scalenus anterior
Explanation
The phrenic nerve, formed mainly from C4 with contributions from C3 and C5, descends vertically on the anterior surface of scalenus anterior, covered by prevertebral fascia, before entering the thorax. Injury here paralyses the ipsilateral hemidiaphragm. The brachial plexus roots emerge between scalenus anterior and scalenus medius, so option D describes the interscalene corridor taken by the plexus and subclavian artery, not the phrenic nerve.
Reference: Gray's Anatomy for Students, 4th ed.
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Written and medically reviewed by the StethoPrep medical team.