In HIV, a ring-enhancing lesion is mainly toxoplasmosis or primary CNS lymphoma. The sources describe toxoplasmosis as multiple ring-enhancing lesions (usually basal ganglia) and the commonest neurological AIDS illness, and mention CNS lymphoma as the other HIV mass lesion. The stem describes a single 'mass', which favours lymphoma (C) — the cohort's corrected key — but the material never states the single-vs-multiple rule outright, and toxoplasmosis (B) is the commoner cause.

• Staphylococcal abscess is unlikely without a source.