For suspected infectious mononucleosis in a young patient, which test is diagnostic?

• A. Throat culture
• B. Blood culture
• C. Heterophile test
• D. Lymph node biopsy

Answer: (C)

Diagnosing suspected infectious mononucleosis is best done with a heterophile antibody test (Monospot). This test detects antibodies produced in response to EBV infection that cause agglutination of animal red blood cells. It works best in adolescents and young adults and typically becomes positive after about a week of symptoms. A negative result doesn’t completely rule out mono, especially early in illness or in younger children, in which case EBV-specific serologies can help confirm. The other options aren’t diagnostic for mono: throat culture targets streptococcus, blood culture looks for bacteria in the blood, and lymph node biopsy isn’t used to diagnose mono.