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::'''Incorrect:''' This is a case of atypical pneumoniae. Streptococcus Pneumoniae is the most common cause of community acquired pneumoniae(~50% of cases). presents in older or immunocompromised individuals and is more acute/severe. |
| Answer B | AnswerB::Staphylococcus Aureus |
| Answer B Explanation | [[AnswerBExp::Incorrect: The patient in this vignette has a case of atypical (walking) pneumoniae. 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::Correct: 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::Incorrect: 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::Incorrect: 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 Pneumonia 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. Educational Objective: Mycoplasma Pneumonia is the most common cause of atypical pneumonia. References: First Aid 2014 page 145 Note: This question is linked to WBR0045,WBR0046 |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Pneumonia, WBRKeyword::Bacteria, WBRKeyword::Respiratory, WBRKeyword::Microbiology, WBRKeyword::Linked |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |