Infectious mononucleosis
USMLE Step 1Difficulty: easyLast updated:
A 17-year-old girl presents with 8 days of fever, severe sore throat, and fatigue. Examination shows exudative tonsillopharyngitis, palatal petechiae, tender posterior cervical lymphadenopathy, and a palpable spleen tip. A rapid streptococcal antigen test is negative. Complete blood count shows lymphocytosis with many atypical lymphocytes. She has no risk factors for HIV infection.
Which of the following is the most appropriate initial test to confirm the most likely diagnosis?
Answer and explanation
Correct answer: D. Heterophile antibody (monospot) test
Fever, exudative pharyngitis, posterior cervical lymphadenopathy, splenomegaly, and atypical lymphocytosis in an adolescent suggest infectious mononucleosis caused by Epstein-Barr virus (EBV). The source notes that atypical lymphocytes also occur in other infections, including HIV, CMV, and hepatitis B, so serologic confirmation is needed. Two serologic tests are used: heterophile antibody testing with agglutination card (monospot) tests, and EBV-specific antibody testing. Heterophile antibodies are present in 85% to 90% of adolescents and adults with infectious mononucleosis but in only about 50% of children aged 2 to 5 years. This makes the monospot a reasonable first confirmatory test in this patient. EBV-specific serologies are used when the heterophile test is negative but suspicion remains. Severe complications include airway obstruction and splenic rupture.
Each option explained
- A. Cold agglutinin titerIncorrect. Cold agglutinins are associated with Mycoplasma pneumoniae infection.
- B. Peripheral smear for schistocytesIncorrect. Schistocytes indicate microangiopathic hemolysis and are not part of the mononucleosis workup.
- C. Blood culturesIncorrect. Blood cultures do not detect EBV and are not indicated for this presentation.
- D. Heterophile antibody (monospot) testCorrect. The heterophile (monospot) test is positive in 85–90% of adolescents and adults with infectious mononucleosis.
- E. Antistreptolysin O titerIncorrect. The ASO titer documents prior streptococcal infection (e.g., for rheumatic fever) and does not confirm EBV.
Key point
Confirm suspected EBV mononucleosis in adolescents with a heterophile (monospot) test; it is often falsely negative in young children.
From the source
“heterophile antibodies are present in only 50% of patients aged 2 to 5 years and in between 85 to 90% of adolescents and adults with infectious mononucleosis”