This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuroanatomy 3, Subject name: Anatomy
Injury to which of the following structures causes constricted pupils even when room lighting is dim? 
Select and explain. 
A. Trochlear nerve, B. Superior cervical ganglion, C. Oculomotor nerve, D. Ophthalmic nerve
B: Superior cervical ganglion
Answer: B: Superior cervical ganglion 
Explanation: Postganglionic sympathetic fibers that originate from the superior cervical ganglion and innervate the dilator pupillae (radial muscles of the iris) are damaged. So the pupil remains small in a dimly lit room. Other nerves mentioned contain no sympathetic fibers, but the oculomotor nerve contains preganglionic parasympathetic fibers.