A 70-year-old man with a history of hypertension, porcine aortic valve replacement, and chronic obstructive pulmonary disease began having dizziness and confusion. He had strange 1-word responses to his son, who took him to a hospital. A computed tomography scan was performed, showing a subarachnoid hemorrhage. There was no neurosurgeon at that hospital; therefore, the patient was transferred the next morning to our institution. The patient's son reported a remote history of falling and the patient hitting his head on a cabinet. The patient denied any headaches, nausea, vomiting, vision changes, or seizures, but he confirmed having a productive cough for the last 2 weeks for which he had been placed on levofloxacin. He smoked 1.5–2 packs of cigarettes a day. His physical examination showed no abnormal neurologic signs, and he was alert and oriented without acute distress. Laboratory tests revealed an increased white blood cell count, abnormal coagulation, and acute renal failure (Table 1). The patient received care and various medications (amiodarone, atorvastatin, azithromycin, benzocaine, citalopram, clonazepam, fluticasone, levalbuterol, metoprolol, and ropinirole) at admission to control his blood pressure, cough, throat pain, and subarachnoid hemorrhage. View this table: Table 1. Laboratory tests at admission. Two days later (day 3), he developed fever (103.7 °F/39.8 °C) and atrial fibrillation, appeared very lethargic, and complained of mild shortness of breath, but he denied any chest pain, nausea, vomiting, or chills. His renal function had improved. His blood culture from the day of admission was positive for gram-positive cocci, and the physician was concerned for possible endocarditis. The patient was thus given vancomycin and cefepime, and he was scheduled for a transesophageal echocardiography (TEE)2 procedure the next morning (day 4). ### QUESTIONS TO CONSIDER 1. What clinical signs and laboratory results lead to a diagnosis of methemoglobinemia? 2. What can cause methemoglobinemia, and what was the cause for this patient? 3. What methods are available to measure …