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Author William J Gibson (Reviewed by Yazan Daaboul)
Exam Type USMLE Step 1
Main Category Microbiology
Sub Category Genitourinary, General Principles, Infectious Disease
Prompt A 22-year-old man presents to his primary care physician complaining of dysuria and mucopurulent urethral discharge for the past 3 days. He states that he had an unprotected sexual encounter 2 weeks prior to presentation. Gram stain of the urethral swab reveals gram-negative diplococci. Which of the following media may be used to isolate the organism responsible for this patient's condition?
Answer A Thayer-Martin agar
Answer A Explanation [[AnswerAExp::Neisseria gonorrhea is a gram-negative diploccocus whose infection constitutes the second most common sexually transmitted illness. Gonorrhea infection often presents as dysuria accompanied by purulent urethral discharge.]]
Answer B MacConkey’s agar
Answer B Explanation [[AnswerBExp::MacConkey’s agar (MAC) is used to culture a variety of gram-negative organisms. Lactose-fermenting enterics (eg Escherichia coli) will appear as pink colonies.]]
Answer C Tellurite agar
Answer C Explanation [[AnswerCExp::Tellurite agar may be use to culture Corynebacterium diptheriae.]]
Answer D Bordet-Gengou agar
Answer D Explanation [[AnswerDExp::Bordet-Gengou agar is used to culture Bordetella pertussis.]]
Answer E Sabouraud’s agar
Answer E Explanation [[AnswerEExp::Sabouraud’s agar is used to culture fungi.]]
Right Answer A
Explanation [[Explanation::The patient is most likely diagnosed with urethritis, a sexually transmitted infection by Neisseria gonorrhea. Neisseria gonorrhea is a gram negative diplococcus. It is the second most common sexually transmitted infection after Chlamydia trachomatis.

Because both organisms cause dysuria and urethral discharge, they may be difficult to differentiate without culture results. Nonetheless, N. gonorrhea is classically associated with a mucopurulent or purulent urethral discharge, whereas Chlamydia trachomatis is associated with a clearer or a rather mucoid discharge. N. gonorrhea may be cultured with Thayer-Martin media. Due to the high rate of co-infection among patients who present with N. gonorrhea urethritis, proper treatment requires coverage for both N. gonorrhea and C. trachomatis by ceftriaxone and azithromycin, respectively.
Educational Objective: Neisseria gonorrhea is a sexually transmitted infection that often presents as dysuria accompanied by purulent urethral discharge. It grows on Thayer-Martin agar. Treatment consists of ceftriaxone to cover N. gonorrhea, along with azithromycin for concomitant coverage of a presumable Chlamydia trachomatis co-infection.
References: First Aid 2014 page 137]]

Approved Yes
Keyword Sexually transmitted infection, STI, STD, Gonorrhea, Microbiology, Bacteria, Laboratory, Culture
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