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Author [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type USMLE Step 1
Main Category Microbiology
Sub Category Pulmonology
Prompt A 20-year-old woman presents to her primary care physician complaining of dyspnea and malaise, associated with a lingering cough. Her symptoms started one week ago and have been gradually worsening. Nonetheless, she is generally still able to perform her daily activities. She denies abnormal sputum production, chest pain, sore throat, or runny nose. Her past medical history is insignificant. She does not use any medications and has no known allergies. Her blood pressure is 120/70 mmHg, heart rate is 72/min, temperature is 101 °F (38.3 °C), respiratory rate is 22/min, and oxygen saturation is 94% on room air. On physical examination, her cardiac auscultation is unremarkable; but pulmonary auscultation reveals diffuse crackles bilaterally. Which of the following may be the causative agent of this patient's condition?
Answer A Streptococcus pneumoniae
Answer A Explanation Streptococcus pneumoniae is the most common cause of community acquired pneumonia (~50% of cases). It presents in older or immunocompromised individuals and is more acute and severe than this patient's presentation.
Answer B Staphylococcus aureus
Answer B Explanation [[AnswerBExp::Staphylococcus aureus causes a more severe lobar pneumonia. S. aureus is also associated with Staphylococcal scalded skin syndrome, toxic shock syndrome, cellulitis, and food poisoning.]]
Answer C Mycoplasma pneumoniae
Answer C Explanation [[AnswerCExp::The patient in this vignette most likely has an atypical (walking) pneumonia that is most often caused by Mycoplasma pneumoniae.]]
Answer D Moraxella catarrhalis
Answer D Explanation [[AnswerDExp::Moraxella catarrhalis is not a common cause of atypical (walking) pneumonia. Instead, M. catarrhalis is a common cause of otitis media in younger children.]]
Answer E Influenza virus
Answer E Explanation Influenza virus can cause an atypical pneumonia but is a less common cause than Mycoplasma pneumoniae. Patients with influenza virus infection often complain of headache, high-grade fever, chills, dry cough, pharyngeal irritation, and myalgias. It is usually self-limited and persists for approximately 2-8 days.
Right Answer C
Explanation [[Explanation::Atypical pneumonia is a lower respiratory tract infection that often causes non-specific symptoms such as fever, headaches, and myalgia. Atypical pneumonia differs from typical pneumonia in that atypical pneumonia often does not respond to common antibiotics (eg sulfonamides, or beta-lactams), does not show signs of focal consolidation, and does not exhibit leukocytosis on lab work-up. On chest X-ray, patients often show diffuse interstitial infiltrates, which falsely convey the appearance of a more severe infection than the patient’s symptoms suggest. Because patients often have mild symptoms, atypical pneumonia is alternatively referred to as “walking pneumonia.” While walking pneumonia due to Mycoplasma pneumoniae infection is a distinct entity, the more general distinction between typical and atypical pneumonias is largely historical and has more recently been de-emphasized.

Atypical pneumonia is most often caused by Mycoplasma pneumoniae, which usually infects older children and young adults (patients <30 years old). Mycoplasma pneumoniae is a small bacteria characterized by its lack of cell wall. Mycoplasma pneumoniae can be cultured on Eaton's agar. Macrolides are considered the optimal antibiotics for atypical pneumonias due to their coverage for atypical organisms, including M. pneumoniae.
Educational Objective: Mycoplasma pneumoniae is the most common cause of atypical pneumonia in adults. Atypical pneumonias are characterized by their gradual course. Patients usually have sign and symptoms of pneumonia, but do not complain of severe symptoms. Paradoxically, chest xray shows diffuse interstitial infiltrates that do not correlate with the severity of the disease.
References: Rello J, Pop-Vicas A. Clinical review: primary infleunza viral pneumonia. Critical Care. 2009;13(6):235. First Aid 2014 page 145]]

Approved Yes
Keyword Pneumonia, Bacteria, Respiratory, Microbiology, Linked
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