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 physical examination, she has a pulse of 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::This is a case of atypical pneumonia. Streptococcus pneumoniae is the most common cause of community acquired pneumonia (~50% of cases). It presents in older or immunocompromised individuals and is more acute/severe than the case above.
Answer B	AnswerB::Staphylococcus aureus
Answer B Explanation	[[AnswerBExp::The patient in this vignette has a case of atypical (walking) pneumonia. Staphylococcus aureus causes a more severe, usually lobar pneumonia. S. aureus is also associated with scalded skin syndrome, toxic shock syndrome, cellulitis and food poisoning.]]
Answer C	AnswerC::Mycoplasma pneumonia
Answer C Explanation	[[AnswerCExp::The patient in this vignette has an atypical (walking) pneumonia that is most often caused by Mycoplasma pneumonia.]]
Answer D	AnswerD::Moraxella catarrhalis
Answer D Explanation	[[AnswerDExp::Moraxella catarrhalis is not a common cause of atypical (walking) pneumonia. Instead, M. catarrhalis is a common cause of community acquired pneumonia.]]
Answer E	AnswerE::Influenzavirus
Answer E Explanation	[[AnswerEExp::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 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 pneumoniae 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 WBR0045,WBR0046 Educational Objective: Mycoplasma pneumoniae is the most common cause of atypical pneumonia. References: First Aid 2014 page 145]]
Approved	Approved::Yes
Keyword	WBRKeyword::Pneumonia, WBRKeyword::Bacteria, WBRKeyword::Respiratory, WBRKeyword::Microbiology, WBRKeyword::Linked
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::