The patient has primary atypical pneumonia caused by Mycoplasma pneumoniae. This organism is fastidious and difficult to culture in the laboratory, however serodiagnosis can be most helpful. Patients typically produce one or two heterophile antibodies during the course of the infection; one agglutinates human O+ RBCs in the cold (the cold hemagglutinin) while the other causes the agglutination of a strain of Streptococcus salivarius termed strain MG (the Strep MG agglutinins). Klebsiella pneumoniae is readily cultured on routine laboratory media and characteristically produces pneumonia with blood clots in the sputum (red currant jelly sputum), which may be indicative of pulmonary abscess development. Parainfluenza viruses cause croup, which is characterized by a dry, "barking" cough. It is more of a tracheitis, bronchitis, and bronchiolitis than a pneumonitis. No heterophile antibodies are produced in these patients. Respiratory syncytial virus causes an atypical pneumonitis in infants. It is usually diagnosed by the observation of syncytial masses in respiratory secretions. Cold hemagglutinins and Strep MG agglutinins are absent. Ref: Levinson W. (2012). Chapter 23. Mycoplasmas. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.