Correct Answer: D. Infectious Mononucleosis
Infectious Mononucleosis (IM), caused by Epstein-Barr Virus (EBV), produces highly characteristic histopathological findings in lymph nodes that are the basis of this question. On lymph node biopsy, IM shows marked paracortical expansion by large, activated T lymphocytes (immunoblasts) responding to EBV-infected B cells. Crucially, these lymph nodes contain Reed-Sternberg-like (RS-like) cells — large binucleated or multinucleated immunoblasts with prominent "owl-eye" nucleoli — which can mimic Hodgkin lymphoma histologically. This is the classic and high-yield "trap" in pathology: IM lymph node biopsy can be mistaken for Hodgkin lymphoma due to these RS-like cells. The overall architecture shows follicular hyperplasia, paracortical expansion, and sinusoidal infiltration by atypical lymphocytes (Downey cells in peripheral blood; immunoblasts in lymph nodes).
EBV infects B lymphocytes via the CD21 (CR2) receptor, driving polyclonal B-cell proliferation. This triggers a massive cytotoxic T-cell response, producing the atypical lymphocytosis seen in peripheral blood smears and the immunoblastic infiltrate in lymph nodes. The combination of lymphadenopathy, splenomegaly, pharyngitis, and atypical lymphocytosis is the clinical hallmark of IM. Lymph node histology showing RS-like immunoblasts in the correct clinical context is diagnostic of EBV-driven Infectious Mononucleosis.
Why other options are wrong
- A. Influenza — Causes acute necrotizing bronchitis and pneumonia with neutrophilic infiltration. Lymph node changes are non-specific reactive hyperplasia only. No characteristic lymph node cells are associated with influenza.
- B. Measles — Produces Warthin-Finkeldey cells (multinucleated giant cells/syncytia in germinal centers of lymph nodes) during the prodromal phase. These are pathognomonic for measles, not IM. Measles also causes Koplik spots on buccal mucosa. The cells in this question are not Warthin-Finkeldey syncytia.
- C. CMV infection — CMV produces "owl's eye" intranuclear inclusions (large basophilic intranuclear inclusion surrounded by a clear halo) in epithelial cells, endothelial cells, and macrophages. While CMV can cause a mononucleosis-like syndrome, the pathognomonic cytopathic effect is the intranuclear inclusion body, not the RS-like immunoblasts of EBV.