WBR0044

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 examination, her pulse is 72, temperature of 101 F (38.3 C), respiratory rate of 22 and oxygen saturation of 94% on room air. Cardiac examination is normal but respiratory exam reveals diffuse crackles. Which of the following is the most likely causal organism?]]
Answer A	AnswerA::Streptococcus Pneumoniae
Answer A Explanation	AnswerAExp::False. This is a case of atypical pneumoniae. S. Pneumo is the most common cause of typical pneumoniae(~50% of cases) which presents in older or immunocompromised individuals and is more acute/severe.
Answer B	AnswerB::Staphylococcus Aureus
Answer B Explanation	AnswerBExp::False. This is a case of atypical pneumoniae. S. Aureus causes a more severe, usually lobar pneumonia.
Answer C	AnswerC::Mycoplasma Pneumonia
Answer C Explanation	AnswerCExp::True. See explanation.
Answer D	AnswerD::Moraxella Catarrhalis
Answer D Explanation	AnswerDExp::False. Moraxella Catarrhalis is a common cause of typical pneumonia, this is a case of atypical pneumonia.
Answer E	AnswerE::Influenzavirus
Answer E Explanation	AnswerEExp::False. The patient’s diffuse crackles are suggestive of a bacterial infection rather than a Flu-like syndrome caused by influenzavirus. Influenzavirus can cause an atypical pneumonia but is a less common cause than mycoplasma pneumonia.
Right Answer	RightAnswer::C
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 it does not respond to common antibiotics (eg sulfonamides, or beta-lactams), does not show signs of consolidation, nor leukocytosis. On chest X-ray, patients often show 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.” Atypical pneumonia is most often caused by Mycoplasma Pneumonia which usually occurs in 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. Educational Objective: Mycoplasma Pneumonia is the most common cause of atypical pneumonia. Educational Objective: References: ]]
Approved	Approved::Yes
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