WBR0045

Author	PageAuthor::William J Gibson
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 dyspnea and malaise. Over the past week she has experienced increasing dyspnea, bronchitis and fatigue which limits her otherwise vigorous exercise but does not interfere with daily activities. She denies abnormal sputum production and any significant past medical history. On physical examination, she has a pulse of 72 beats/min, temperature of 101 °F (38.3 °C), respiratory rate of 22/min and oxygen saturation of 94% on room air. Cardiac examination is normal but respiratory exam reveals diffuse crackles. Which of the following is true of the most likely causal 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 contains cholesterol
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 species stain positive in acid fast stain, but 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.]]
Answer E	AnswerE::No cell wall
Answer E Explanation	[[AnswerEExp::Mycoplasma pneumoniae is the most common cause of atypical pneumonia and has no cell wall.]]
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 do not respond to common antibiotics (eg sulfonamides, or beta-lactams), do not show signs of consolidation, nor give rise toe leukocytosis. On chest X-ray, patients often show a diffuse 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.” While walking pneumonia due to mycoplasma 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 pneumonia is a mere 0.3 um in size and is the only bacteria which does not possess a cell wall. Mycoplasma pneumoniae can 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 and has no cell wall. References: First Aid 2014 page 145]]
Approved	Approved::Yes
Keyword	WBRKeyword::Microbiology, WBRKeyword::Mycoplasma, WBRKeyword::Bacteria, WBRKeyword::Pneumonia, WBRKeyword::infection
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::