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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor=William J Gibson (Reviewed by {{Rim}})
|QuestionAuthor=William J Gibson (Reviewed by {{Rim}} and Yazan Daaboul)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Microbiology
|MainCategory=Microbiology
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|MainCategory=Microbiology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|MainCategory=Microbiology
|MainCategory=Microbiology
|MainCategory=Microbiology
|MainCategory=Microbiology
Line 20: Line 21:
|MainCategory=Microbiology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|SubCategory=Pulmonology
|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?
|Prompt=A 20-year-old woman presents to her primary care physician complaining of gradually worsening dyspnea, fever, and malaise. Her past medical history is only significant for migraines, for which she occasionally takes over-the-counter NSAIDs. She denies any known allergies to food or medications. Her blood pressure is 116/82 mmHg, heart rate is 72/min, temperature is 101 °F (38.3 °C), and respiratory rate is 24/min. Physical examination is prominent for diffuse crackles over both lung fields. Otherwise, her physical exam is completely normal. , cardiac examination is normal but respiratory exam reveals diffuse crackles.  Which of the following is a characteristic finding of her infectious organism?
|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.
|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 have a milder course, 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 with atypical pneumonia often have a diffuse bilateral 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 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.
Atypical pneumonia is most often caused by [[''Mycoplasma pneumoniae'']], which usually infects older children and young adults less than 30 years of age. ''Mycoplasma pneumonia'' is a small bacteria that does not contain a cell wall. Instead, it contains a cell membrane that has a high sterol content, which decreases its fluidity and supports its osmotically fragile membrane. It is difficult to culture, but may be cultured on Eaton's agar.




Note: This question is linked to WBR0044
Note: This question is linked to WBR0044
|AnswerA=Pili undergo antigenic variation
|AnswerA=Pili undergo antigenic variation
|AnswerAExp=The pili of [[Neisseria gonorrheae]] undergo antigenic variation.
|AnswerAExp=The pili of [[''Neisseria gonorrheae'']] undergo antigenic variation.
|AnswerB=Cell wall contains cholesterol
|AnswerB=Cell wall that contains high sterol content
|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.
|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.
|AnswerC=Acid fast bacilli
|AnswerC=Acid fast bacilli
|AnswerCExp=[[Mycobateria]] species stain positive in acid fast stain, but Mycoplasma are not identified in this manner.
|AnswerCExp=[[''Mycobateria'']] species stain positive in acid fast stain. ''Mycoplasma'' are not identified in this manner.
|AnswerD=Positive silver stain
|AnswerD=Positive silver stain
|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]].
|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'']].
|AnswerE=No cell wall
|AnswerE=Presence of cell membrane with increased osmotic fragility
|AnswerEExp=[[Mycoplasma pneumoniae]] is the most common cause of [[atypical pneumonia]] and has no [[cell wall]].
|AnswerEExp=[[''Mycoplasma pneumoniae'']] is the most common cause of [[atypical pneumonia]] and has no [[cell wall]]. It contains cell membranes with high sterol content that decrease its membrane fluidity and increase its osmotic fragility.
|EducationalObjectives=[[Mycoplasma pneumoniae]] is the most common cause of [[atypical pneumonia]] and has no cell wall.
|EducationalObjectives=[[''Mycoplasma pneumoniae'']] is the most common cause of [[atypical pneumonia]]. It is characterized by the absence of cell wall, and presence of cell membranes with high sterol content that decrease its membrane fluidity and supports its osmotically fragile membrane.
|References=First Aid 2014 page 145
|References=Waites KB, Talkington DF. Mycoplasma pneumoniae and its role as a human pathogen. Clin. Microbiol. Rev. 2004;17(4):697-728.
First Aid 2014 page 145
|RightAnswer=E
|RightAnswer=E
|WBRKeyword=Microbiology, Mycoplasma, Bacteria, Pneumonia, infection
|WBRKeyword=Microbiology, Mycoplasma, pneumoniae, Bacteria, Pneumonia, infection, atypical, walking, cell, membrane, sterol
|Approved=Yes
|Approved=Yes
}}
}}

Revision as of 14:57, 1 August 2014
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
Sub Category SubCategory::Pulmonology
Prompt [[Prompt::A 20-year-old woman presents to her primary care physician complaining of gradually worsening dyspnea, fever, and malaise. Her past medical history is only significant for migraines, for which she occasionally takes over-the-counter NSAIDs. She denies any known allergies to food or medications. Her blood pressure is 116/82 mmHg, heart rate is 72/min, temperature is 101 °F (38.3 °C), and respiratory rate is 24/min. Physical examination is prominent for diffuse crackles over both lung fields. Otherwise, her physical exam is completely normal. , cardiac examination is normal but respiratory exam reveals diffuse crackles. Which of the following is a characteristic finding of her infectious 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 that contains high sterol content
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. 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::Presence of cell membrane with increased osmotic fragility
Answer E Explanation [[AnswerEExp::''Mycoplasma pneumoniae'' is the most common cause of atypical pneumonia and has no cell wall. It contains cell membranes with high sterol content that decrease its membrane fluidity and increase its osmotic fragility.]]
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 have a milder course, 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 with atypical pneumonia often have a diffuse bilateral 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 pneumoniae'', which usually infects older children and young adults less than 30 years of age. Mycoplasma pneumonia is a small bacteria that does not contain a cell wall. Instead, it contains a cell membrane that has a high sterol content, which decreases its fluidity and supports its osmotically fragile membrane. It is difficult to culture, but may 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. It is characterized by the absence of cell wall, and presence of cell membranes with high sterol content that decrease its membrane fluidity and supports its osmotically fragile membrane.
References: Waites KB, Talkington DF. Mycoplasma pneumoniae and its role as a human pathogen. Clin. Microbiol. Rev. 2004;17(4):697-728. First Aid 2014 page 145]]

Approved Approved::Yes
Keyword WBRKeyword::Microbiology, WBRKeyword::Mycoplasma, WBRKeyword::pneumoniae, WBRKeyword::Bacteria, WBRKeyword::Pneumonia, WBRKeyword::infection, WBRKeyword::atypical, WBRKeyword::walking, WBRKeyword::cell, WBRKeyword::membrane, WBRKeyword::sterol
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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