The etiologic agent of primary atypical pneumonia is which organism?

• A. Chlamydia trachomatis
• B. Chlamydiophila psittaci
• C. Mycoplasma pneumoniae
• D. Ureaplasma urealyticum

Answer: (C)

Primary atypical pneumonia is most often due to organisms that produce milder, diffuse interstitial disease rather than a typical lobar pneumonia. The classic culprit in this setting is Mycoplasma pneumoniae, especially in teenagers and young adults, causing symptoms like a dry cough with low-grade fever and malaise and producing patchy or interstitial infiltrates on imaging. A key feature of this organism is that it lacks a cell wall, so it does not Gram stain well and is not susceptible to beta-lactam antibiotics; it’s typically treated with macrolides, doxycycline, or certain fluoroquinolones. While Chlamydophila psittaci can cause atypical pneumonia (often from bird exposure) and Chlamydia trachomatis can be involved in neonatal or specific mucosal infections, they are not the most common causes of adult primary atypical pneumonia. Ureaplasma urealyticum is mainly linked to urogenital infections and some neonatal pneumonia, not the classic adult presentation of primary atypical pneumonia.