CNS infections

A 41-year-old male with a history of Pneumocystis jirovecii pneumonia is found to have multiple ring-enhancing lesions on brain CT. Which of the following is most likely responsible for this patient's abnormal scan?

A 15-year-old boy is brought to the Emergency department by ambulance from school. He started the day with some body aches and joint pain but then had several episodes of vomiting and started complaining of a terrible headache. The school nurse called for emergency services. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He is a good student and enjoys sports. At the hospital, his blood pressure is 120/80 mm Hg, heart rate is 105/min, respiratory rate is 21/min, and his temperature is 38.9°C (102.0°F). On physical exam, he appears drowsy with neck stiffness and sensitivity to light. Kernig’s sign is positive. An ophthalmic exam is performed followed by a lumbar puncture. An aliquot of cerebrospinal fluid is sent to microbiology. A gram stain shows gram-negative diplococci. A smear is prepared on blood agar and grows round, smooth, convex colonies with clearly defined edges. Which of the following would identify the described pathogen?