WBR0053

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Author [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type USMLE Step 1
Main Category Microbiology, Pathology
Sub Category General Principles, Infectious Disease
Prompt A microbiologist has been given a number of unknown clinical samples and is attempting to visualize the organisms by gram stain. Samples from which patient population will he be unable to visualize with this method?
Answer A Sexually active college dormitory students
Answer A Explanation [[AnswerAExp::Nisseria meningitidis is a gram-negative dipoloccus.]]
Answer B Adult patients with a history of alcoholism
Answer B Explanation [[AnswerBExp::Klebsiella pneumoniae is a gram-negative rod.]]
Answer C Infants of pregnant women who were not screened during 3rd trimester
Answer C Explanation [[AnswerCExp::Group B streptococcus is a gram-positive coccus.]]
Answer D Patients with recent exposure to contaminated air conditioning systems
Answer D Explanation [[AnswerDExp::Legionella pneumophila is an intracellular organism which gram-stains poorly. The stain of choice for Legionella is a silver stain.]]
Answer E Elderly patients with osteoarthritis who recently underwent a total hip replacement by prosthesis
Answer E Explanation [[AnswerEExp::Staphyloccocus epidermidis is a gram-positive coccus.]]
Right Answer D
Explanation [[Explanation::Legionella pneumophila is an intracellular organism which gram-stains poorly. The stain of choice for Legionella is a silver stain. Legionella causes atypical pneumonia and is transmitted by aerosols from contaminated water, such as air conditioning systems or cruises in lakes and rivers. Legionella infection can cause Legionnaires' disease, which is a severe illness in older individuals. However, it may also cause a milder manifestation termed Pontiac fever. The treatment of choice for Legionella infections is a macrolide (e.g. erythromycin).

Educational Objective: Legionella pneumophila is an intracellular organism which gram-stains poorly. The stain of choice for Legionella is a silver stain.
References: First Aid 2014 page 121]]

Approved Yes
Keyword Bacteria, Microbiology, Legionella, Gram stain, Stain, silver, air condition, air conditioning
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