October 13, 2010

USMLE 1 Question 7

A 35 year old man has a three day history of a productive cough. His x-ray shows evidence of bilateral consolidation. A sputum culture grows a gram positive organism that is lysed by ethyl hydrocupriene (Optochin).

These features are characteristic of

a) Streptococcus pneumoniae
b) Bacteroides fragilis
c) Coxiella burnetii
d) Escherichia coli
e) Haemophilus influenzae






ANSWER
The correct answer is A


Explanation
The optochin test is a presumptive test that is used to identify strains of Streptococcus pneumoniae.

Optochin (ethyl hydrocupreine) disks are placed on inoculated blood agar plates. Because S. pneumoniae is not optochin resistant, a zone of inhibition will develop around the disk where the bacteria have been lysed. This zone is typically 14mm from the disk or greater.

Streptococcus pneumoniae is a gram-positive coccus that in clinical specimens often appears in pairs with tapered ends (lancet shaped), although it grows in short chains on nutrient broth media. It is a relatively fastidious facultative anaerobe and requires increased carbon dioxide for optimum recovery from clinical specimens.

Susceptibility to ethyl hydrocupriene hydrochloride (Optochin) distinguishes pneumococci from other streptococci in the clinical microbiology laboratory.

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