A 35-year-old man is brought to the ICU following severe head trauma with a Glasgow Coma Scale of 3. Clinical examination confirms absent pupillary light reflex, absent corneal reflex, absent oculocephalic reflex, and absent gag reflex. The apnea test cannot be performed safely due to severe facial trauma and cervical spine injury precluding reliable assessment. The structure marked **D** in the diagram represents an ancillary test for brain death determination. Which of the following ancillary tests would be MOST appropriate to confirm brain death in this patient with confounded clinical examination?