A 5-year-old unvaccinated child presents with a 3-day history of fever, cough, coryza, and conjunctivitis. On day 4, you notice tiny white spots with a red base on the buccal mucosa opposite the lower molars, followed by a maculopapular rash starting behind the ears. Which of the following is the most characteristic laboratory finding?

• A Leukocytosis with neutrophilia
• B Thrombocytosis
• C Elevated serum procalcitonin
• D Leukopenia with relative lymphocytosis ✓

Explanation

This presentation is classic for measles (rubeola): the prodrome of 3 Cs (cough, coryza, conjunctivitis) followed by Koplik spots (pathognomonic white spots on buccal mucosa) and then a maculopapular rash beginning at the hairline/behind ears and spreading downward. Measles is caused by a paramyxovirus and characteristically produces leukopenia with relative lymphocytosis, which also increases susceptibility to secondary bacterial infections. This leukopenia distinguishes it from bacterial infections causing leukocytosis.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.