A 29-year-old man comes to the emergency department due to lower extremity weakness. The patient has not been feeling well over the past several days and has had worsening back pain. When he woke up this morning, he noticed weakness in both legs and had to have a friend help him walk to the car. The patient has a history of HIV, which was acquired via shared needles, but does not take antiretroviral therapy consistently. The patient still uses injection drugs occasionally but no longer shares needles. Temperature is 38 C (100.4 F), blood pressure is 116168 mm Hg, and pulse is 98/min. The lungs are clear on auscultation, and heart sounds are normal with no murmurs. Abdominal examination shows a distended bladder. The back is tender to palpation over the lower thoracic region. Neurological examination reveals 315 bilateral lower extremity muscle strength with upgoing bilateral plantar reflexes. Skin examination shows scattered folliculitis and no lower extremity edema.