WBR0045

Author	[[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Pulmonology
Prompt	[[Prompt::A 20-year-old woman presents to her primary care physician complaining of gradually worsening dyspnea, fever, and malaise. Her past medical history is only significant for migraines, for which she occasionally takes over-the-counter NSAIDs. She denies any known allergies to food or medications. 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 prominent for diffuse crackles over both lung fields. Otherwise, her physical exam is completely normal. , cardiac examination is normal but respiratory exam reveals diffuse crackles. Which of the following is a characteristic finding of her infectious organism?]]
Answer A	AnswerA::Pili undergo antigenic variation
Answer A Explanation	[[AnswerAExp::The pili of Neisseria gonorrheae undergo antigenic variation.]]
Answer B	AnswerB::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	AnswerC::Acid fast bacilli
Answer C Explanation	[[AnswerCExp::Mycobateria spp. stain positive in acid fast stain. Mycoplasma are not identified in this manner.]]
Answer D	AnswerD::Positive silver stain
Answer D Explanation	[[AnswerDExp::The causal organism in this vignette is Mycoplasma pneumoniae which is not detected by silver stain. Organisms which are positive on silver stain include Pseudomonas, Legionella, Leptospira, H. pylori, and fungi such as Pneumocystis, and Candida spp.]]
Answer E	AnswerE::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	RightAnswer::E
Explanation	[[Explanation::The patient is suffering from a case of atypical pneumonia, a respiratory infection which often causes systemic 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), do not show signs of focal consolidation, nor give rise to leukocytosis. On chest X-ray, patients with atypical pneumonia often have a diffuse bilateral interstitial infiltrate which conveys 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.” Atypical pneumonia is most often caused by ''Mycoplasma pneumoniae'', which usually infects older children and young adults less than 30 years of age. Mycoplasma pneumonia 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. Note: This question is linked to WBR0044 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 supports 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	Approved::Yes
Keyword	WBRKeyword::Microbiology, WBRKeyword::Mycoplasma, WBRKeyword::pneumoniae, WBRKeyword::Bacteria, WBRKeyword::Pneumonia, WBRKeyword::infection, WBRKeyword::atypical, WBRKeyword::walking, WBRKeyword::cell, WBRKeyword::membrane, WBRKeyword::sterol
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::