A 15 year-old male with URI is now presenting with bilateral hilar infiltrates on chest X-ray, normal WBC, and elevated cold agglutinin titer. Most likely diagnosis?

• A. Viral pneumonia
• B. Streptococcus pneumoniae
• C. Mycoplasma pneumoniae
• D. Bordetella pertussis

Answer: (C)

This question hinges on recognizing atypical pneumonia patterns in adolescents and the immune clue provided by the cold agglutinin titer. Mycoplasma pneumoniae typically causes a walking pneumonia in teenagers, often developing after a recent URI. The chest X-ray classically shows bilateral, often perihilar or interstitial infiltrates rather than a single focal lobar consolidation. The white blood cell count is usually normal or only mildly elevated, reflecting a less pus-forming infection compared with typical bacterial pneumonia. The elevated cold agglutinin titer is a classic trick: Mycoplasma can trigger IgM antibodies that agglutinate red blood cells at cold temperatures, a nonspecific clue you may see in this infection.

Why the other possibilities fit less well: viral pneumonia can produce similar interstitial patterns, but the presence of cold agglutinins strongly points toward Mycoplasma rather than a generic viral etiology. Streptococcus pneumoniae typically causes focal lobar consolidation with a higher WBC count, not the bilateral perihilar pattern and normal WBC seen here. Bordetella pertussis presents with the characteristic whooping cough and lymphocytosis rather than the radiographic pattern described.

So, the most likely diagnosis is Mycoplasma pneumoniae infection, the classic cause of atypical pneumonia in adolescents with bilateral perihilar infiltrates, normal WBC, and cold agglutinins.