A 19-year-old student has dry cough, low-grade fever and malaise for 10 days with patchy bilateral infiltrates. Sputum culture on ordinary media is sterile. Which laboratory approach establishes the diagnosis?

• A Sputum Gram stain showing gram-negative diplococci
• B Blood culture held for 21 days
• C Urinary antigen test for Streptococcus pneumoniae
• D PCR or serology for Mycoplasma pneumoniae, since it lacks a cell wall and will not grow on routine media ✓

Explanation

Mycoplasma pneumoniae lacks a peptidoglycan cell wall, so it does not stain by Gram method and cannot grow on standard bacteriological media; diagnosis rests on PCR or on paired serology, often supported by a rising cold agglutinin titre. Pneumococcal urinary antigen excludes this organism, and prolonged blood culture targets fastidious bacteria such as HACEK group, not cell-wall-deficient mollicutes.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.