A 22-year-old man is struck in the left lower chest by a cricket ball. Two hours later he has constant pain at the tip of the left shoulder, worse when he lies flat. The shoulder is not tender, moves fully in every direction without pain, and neck movements do not alter it. His pulse is 110 per minute and the left upper abdomen is tender.
Pain at the tip of this man's left shoulder is carried by afferent fibres travelling in which one of the following nerves?