WBR0045

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Author [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul, M.D.)]]
Exam Type USMLE Step 1
Main Category Microbiology
Sub Category Pulmonology
Prompt A 25-year-old woman presents to her primary care physician with complaints of worsening dyspnea, dry cough, and myalgia for the past 8 days. Her past medical history is significant for migraines, for which she occasionally takes over-the-counter NSAIDs. She denies smoking, drinking alcohol, or illicit drug use. She has no known allergies. Her blood pressure is 116/82 mmHg, heart rate is 72/min, temperature is 101 °F (38.3 °C), and respiratory rate is 24/min. Physical examination is remarkable for diffuse crackles over both lung fields. Chest xray shows bilateral bronchial wall thickening and centrilobular nodules. Which of the following is a characteristic finding of the organism responsible for this patient's symptoms?
Answer A Pili undergo antigenic variation
Answer A Explanation [[AnswerAExp::The pili of Neisseria gonorrheae undergo antigenic variation.]]
Answer B Cell wall that contains high sterol content
Answer B Explanation [[AnswerBExp::Mycoplasma pneumoniae does not have a cell wall. Its cell "membrane" however, incorporates sterol compounds such as cholesterol. It is the only bacterial membrane which contains cholesterol.]]
Answer C Acid fast bacilli
Answer C Explanation [[AnswerCExp::Mycobateria spp. stain positively in acid fast stain. Mycoplasma spp. are not identified using acid fast stain.]]
Answer D Positive silver stain
Answer D Explanation [[AnswerDExp::The infectious agent of this patient's atypical pneumonia is Mycoplasma pneumoniae, which is not detected by silver stain. Organisms which are positive on silver stain include Pseudomonas spp., Legionella spp., Leptospira spp., H. pylori, and fungi such as Pneumocystis jirovecii, and Candida spp.]]
Answer E Presence of cell membrane with increased osmotic fragility
Answer E Explanation [[AnswerEExp::Mycoplasma pneumoniae is the most common cause of atypical pneumonia and has no cell wall. It contains cell membranes with high sterol content that decrease its membrane fluidity and increase its osmotic fragility.]]
Right Answer E
Explanation [[Explanation::Atypical pneumonia is a lower respiratory tract infection which often causes non-specific symptoms such as fever, headache, and myalgia. Atypical pneumonia differs from typical pneumonia in that atypical pneumonias often have a milder course, do not respond to common antibiotics (eg sulfonamides, or beta-lactams), and do not show signs of focal consolidation, and do not exhibit leukocytosis. Chest xray of patients with atypical pneumonia classically shows bilateral bronchial wall thickening and centrilobular nodules, which may falsely convey the appearance of a more severe infection. Because patients often have mild symptoms, atypical pneumonia is alternatively referred to as “walking pneumonia.”

Atypical pneumonia is most often caused by Mycoplasma pneumoniae, which usually infects older children and young adults less than 30 years of age. Mycoplasma pneumoniae is a small bacteria that does not contain a cell wall. Instead, it contains a cell membrane that has a high sterol content, which decreases its fluidity and supports its osmotically fragile membrane. It is difficult to culture, but may be cultured on Eaton's agar.
Educational Objective: Mycoplasma pneumoniae is the most common cause of atypical pneumonia. It is characterized by the absence of cell wall, and presence of cell membranes with high sterol content that decrease its membrane fluidity and help support its osmotically fragile membrane.
References: Waites KB, Talkington DF. Mycoplasma pneumoniae and its role as a human pathogen. Clin. Microbiol. Rev. 2004;17(4):697-728. First Aid 2014 page 145.]]

Approved Yes
Keyword Microbiology, Mycoplasma, pneumoniae, Bacteria, Pneumonia, infection, atypical, walking, cell, membrane, sterol
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