Review Question - QID 216376

A 45-year-old man with a history of infection with human immunodeficiency virus (HIV) who is not compliant with antiretroviral therapy (ART) is brought to an HIV clinic by his partner. The patient’s partner reports that the patient has become increasingly apathetic, irritable, and lacking in motivation. The patient also reports frequently getting lost while walking around his neighborhood and having difficulty concentrating. He has no other significant past medical history and takes no medications. His temperature is 98.1°F (36.7°C), blood pressure is 110/75 mmHg, pulse is 70/min, and respirations are 14/min. On physical exam, the patient is unable to rapidly flip his hand back and forth on a table and has a markedly slowed timed gait test. To further evaluate the patient, laboratory testing is done as below:

CD4+ T-lymphocyte count: 90 cells/mm^3
Methylmalonic acid: 105 nmol/L (normal 70-270 nmol/L)
Homocysteine: 9 micromol/L (normal 5-15 micromol/L)

An MRI of the brain is seen in Figure A. Which of the following is the most likely diagnosis?