PLE-1551

A 78-year-old male car mechanic presents with a 24-hour history of progressive headache, vomiting, and dizziness. Upon examination, he is awake but appears confused and has generalized weakness. His carboxyhemoglobin level is 18%. The diagnosis for the patient is carbon monoxide poisoning. Carbon monoxide is dangerous because it is a potent inhibitor of which complex in the electron transport chain?