An HIV-positive patient with CD4 count below 100 cells/microliter has multiple ring-enhancing lesions in the basal ganglia on MRI. Which investigation best differentiates cerebral toxoplasmosis from primary CNS lymphoma when biopsy is not feasible?

• A Thallium-201 SPECT showing increased uptake in lymphoma ✓
• B CSF cytology for atypical lymphocytes
• C Serum Toxoplasma IgG antibody titer
• D CSF opening pressure measurement

Explanation

Thallium-201 SPECT shows avid uptake in primary CNS lymphoma because of its high cellular density, whereas toxoplasmic abscesses are photopenic. This distinction guides whether to start empirical antitoxoplasma therapy or proceed to biopsy. B positive serum Toxoplasma IgG only indicates prior exposure and is common in the general population, so it cannot confirm active disease. CSF cytology rarely yields diagnostic lymphoma cells, and opening pressure is nonspecific.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.