Infectious Mononucleosis: The Daily PANCE Blueprint

The answer is C. Infectious mononucleosis

Profound fatigue, exudative pharyngitis, posterior cervical lymphadenopathy, splenomegaly, and atypical lymphocytes in a young adult is infectious mononucleosis, nearly always Epstein-Barr virus. Two features in this stem separate it cleanly from strep throat. The first is posterior cervical adenopathy — streptococcal pharyngitis produces anterior cervical nodes, and the posterior chain plus splenomegaly should move mono to the front of your list. The second is that rash after amoxicillin. It appears in a large majority of mono patients given aminopenicillins, it is not a true penicillin allergy, and it should never be recorded as one — it is a transient immune-mediated eruption that resolves and does not preclude future penicillin use. Mislabeling it follows patients for decades. EBV infects B lymphocytes through CD21; the atypical lymphocytes on the smear are the reactive CD8 T cells responding to them, not infected cells. Diagnosis is the heterophile antibody (Monospot) test, which is convenient but can be falsely negative in the first week of illness and in young children — when the clinical picture fits and the Monospot is negative, order EBV-specific serology (viral capsid antigen IgM and IgG, EBNA). Management is supportive: rest, fluids, and analgesia. Corticosteroids are not routine and are reserved for airway compromise from tonsillar hypertrophy, severe hemolytic anemia, or severe thrombocytopenia. The counseling point that matters most is splenic rupture — rare but potentially fatal, and the reason to avoid contact sports and heavy lifting for at least three to four weeks and until the splenomegaly resolves. Complications worth knowing include hemolytic anemia, thrombocytopenia, hepatitis, and the association with Burkitt lymphoma, nasopharyngeal carcinoma, and post-transplant lymphoproliferative disease. Sorting the distractors: streptococcal pharyngitis gives anterior nodes, no splenomegaly, no atypical lymphocytes, and a positive rapid antigen test; acute HIV infection is the most important mimic and can look nearly identical with fever, pharyngitis, adenopathy, and rash — which is exactly why an HIV RNA or fourth-generation antigen/antibody test belongs in the work-up of a mono-like illness, particularly with a compatible exposure history; cytomegalovirus mononucleosis is heterophile-negative and causes far less pharyngitis and adenopathy; and diphtheria produces a gray, adherent pseudomembrane that bleeds when scraped, with a bull-neck appearance and systemic toxicity, and is vanishingly rare in vaccinated populations.