Review Question - QID 213212

A 42-year-old man presents to his primary care physician complaining of subjective fever, cough, and night sweats. He states that over the past 2 months he has “not felt like myself.” He has lost 12 lbs over this time period. Two weeks ago, he started experiencing night sweats and cough. This morning he decided to take his temperature and reports it was “high.” He has a history of HIV and admits to inconsistently taking his anti-retrovirals. A chest radiograph reveals a cavitary lesion in the left upper lobe. An interferon-gamma release assay is positive, and the patient is started on appropriate antimicrobial therapy. A month later he is seen in clinic for follow-up. Lab work is obtained, as shown below:

Leukocyte count: 11,000/mm^3 with normal differential
Hemoglobin: 9.2 g/dL
Platelet count: 400,000/mm^3
Mean corpuscular volume (MCV): 75 µm^3

Based on these results, a peripheral smear is sent and shows Pappenheimer bodies. Which of the following is the most accurate test for the patient’s diagnosis?