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 {{WBRQuestion
|QuestionAuthor=William J Gibson (Reviewed by  {{Rim}} and  {{YD}})
|ExamType=USMLE Step 1
|MainCategory=Microbiology
|SubCategory=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?
|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.
|AnswerA=Pili undergo antigenic variation
|AnswerAExp=The pili of ''[[Neisseria gonorrheae]]'' undergo antigenic variation.
|AnswerB=Cell wall that contains high sterol content
|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.
|AnswerC=Acid fast bacilli
|AnswerCExp=''[[Mycobateria spp.]]'' stain positively in acid fast stain. ''Mycoplasma spp.'' are not identified using acid fast stain.
|AnswerD=Positive silver stain
|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.]]''
|AnswerE=Presence of cell membrane with increased osmotic fragility
|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.
|EducationalObjectives=''[[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.
|RightAnswer=E
|WBRKeyword=Microbiology, Mycoplasma, pneumoniae, Bacteria, Pneumonia, infection, atypical, walking, cell, membrane, sterol
|Approved=Yes
}}