WBR0044

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, MainCategory::Neurobiology
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 gradually increasing dyspnea, bronchitis and fatigue which limit her otherwise vigorous exercise but do not interfere with her daily activities. She denies abnormal sputum production or any significant past medical history. Her blood pressure is 120/70 mmHg, heart rate is of 72/min, temperature is 101 F (38.3 C), respiratory rate is 22/min, and oxygen saturation is 94% on room air. On physical examination, her cardiac auscultation is normal; but pulmonary auscultation reveals diffuse crackles bilaterally. Which of the following is true of 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 pneumoniae''
Answer C Explanation	[[AnswerCExp::The patient in this vignette most likely 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::Influenza virus
Answer E Explanation	[[AnswerEExp::The patient’s diffuse crackles are suggestive of a bacterial infection rather than a flu-like syndrome caused by influenza virus. Influenza virus can cause an atypical pneumonia but is a less common cause than Mycoplasma pneumoniae.]]
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 focal consolidation, nor give rise to 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 pneumoniae'' 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 small bacteria characterized by its lack of cell wall. Mycoplasma pneumoniae can be cultured on Eaton's agar. Educational Objective: Mycoplasma pneumoniae is the most common cause of atypical pneumonia. Atypical pneumonias are characterized by their gradual course. Patients usually have sign and symptoms of pneumonia, but do not complain of severe symptoms. Paradoxically, chest xray shows diffuse interstitial infiltrates that do not correlate with the severity of the disease. 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::