Which test demonstrates medullary function?

• A. Pupillary reflex test
• B. Corneal reflex test
• C. Cold caloric test
• D. Apnea test

Answer: (D)

The medullary respiratory center is what drives automatic breathing. The apnea test directly checks this function by removing the ventilatory drive and watching whether the patient initiates breaths as carbon dioxide rises. If there are no spontaneous respirations when PaCO2 increases to a threshold (and the patient remains oxygenated), this demonstrates loss of medullary respiratory drive, indicating medullary dysfunction.

Pupillary reflex tests midbrain pathways controlling pupil size, not the medulla. Corneal reflex involves cranial nerves V and VII and brainstem circuits but doesn’t specifically assess the medullary respiratory center. Cold caloric testing evaluates brainstem–vestibular–oculomotor pathways and overall brainstem integrity, not the medullary centers that generate breathing. Therefore, the apnea test is the best choice for demonstrating medullary function.