Mycoplasma pneumoniae causes atypical pneumonia. Which of the following laboratory findings is a DIRECT consequence of M. pneumoniae infection and helps distinguish it from bacterial pneumonia?
- A Elevated cold agglutinins (IgM antibodies against RBC I antigen, induced by M. pneumoniae cross-reactive antigens) causing a positive cold agglutinin test at 4°C ✓
- B High blood neutrophil count (>15,000/µL) with band forms (left shift)
- C Positive Gram stain of sputum showing gram-negative diplococci
- D Growth on blood agar within 24 hours allowing early culture identification
Explanation
M. pneumoniae infection induces cross-reactive IgM antibodies that agglutinate human RBCs in the cold (0–4°C) by binding the I antigen — these are cold agglutinins (Landsteiner cold hemagglutinins). A titre ≥1:64 or four-fold rise is considered significant. Cold agglutinins occur in ~50% of M. pneumoniae infections; they are also positive in infectious mononucleosis and some other infections. M. pneumoniae lacks a cell wall (no Gram stain), cannot be cultured on ordinary media (requires SP4 medium), and typical pneumonia causes only mild leucocytosis with lymphocytosis, not neutrophilia.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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