**Core Concept**
Foam cells are macrophages that have taken up large amounts of lipids, often seen in atherosclerosis or chronic inflammatory conditions. In viral infections, their presence is a hallmark of specific viruses that induce persistent infection and immune cell activation, particularly in lymphoid tissues.

**Why the Correct Answer is Right**
Epstein-Barr virus (EBV) is known to establish latent infection in B lymphocytes and can lead to the formation of lymphoid follicles with abundant macrophages. These macrophages engulf viral particles and debris, becoming foam cells due to lipid accumulation. This is especially evident in infectious mononucleosis, where EBV causes mononuclear cell infiltration and histopathological findings of foamy macrophages in the tonsils and lymph nodes.

**Why Each Wrong Option is Incorrect**
Option A: Measles causes exanthem and lymphadenopathy but does not lead to foam cell formation in histology.
Option C: Molluscum contagiosum is a skin virus causing pearly papules; it does not involve lymphoid tissue or macrophage transformation.
Option D: RSV primarily affects the lower respiratory tract and causes bronchiolitis; it does not induce foam cells in lymphoid tissues.

**Clinical Pearl / High-Yield Fact**
EBV is the only virus consistently associated with histopathological foam cells in lymphoid tissues—this is a key feature in diagnosing infectious mononucleosis and differentiating it from other viral infections.

✓ Correct Answer: B. EBV