WBR0047

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::General Principles, SubCategory::Infectious Disease
Prompt	[[Prompt::A 25-year-old man presents to his primary care physician for a painless lesion on his penis (pictured below). He reports unprotected vaginal intercourse with a sex worker while on a trip to Las Vegas approximately one month ago. Physical exam is negative for inguinal adenopathy or vesicular lesions on other areas of the body. The most likely causal organism can be identified by which of the following stains? ]]
Answer A	AnswerA::Giemsa stain
Answer A Explanation	[[AnswerAExp::Giemsa stain is used to identify Borrelia, Plasmodium, and Chlamydia.]]
Answer B	AnswerB::Carbol fuschin
Answer B Explanation	[[AnswerBExp::Carbol fuschin (also known as Ziehl-Neelsen stain) is used to identify acid-fast organisms, such as M. tuberculosis. Syphilis is not an acid-fast organism.]]
Answer C	AnswerC::Silver stain
Answer C Explanation	[[AnswerCExp::Silver stain is used to identify fungi such as Pneumocystis jirovecii (previously Pneumocystic carinii). Pneumocystis jirovecii classically causes pneumonia in individuals with advanced HIV infection.]]
Answer D	AnswerD::Fluorescent antibody
Answer D Explanation	[[AnswerDExp::Treponema pallidum can optimally be identified using darkfield microscopy. Another diagnostic test is the Fluorescent Treponemal Antibody Absorption (FTA-Abs assay). This assay is more specific than the Rapid Plasmin Reagin (RPR) or VDRL tests for syphilis, and may thus be used to diagnosis syphilis.]]
Answer E	AnswerE::India Ink
Answer E Explanation	[[AnswerEExp::India ink stain is used to visualize Cryptococcus neoformans. Cryptococcus neoformans is classically associated with meningitis in HIV-positive individuals.]]
Right Answer	RightAnswer::D
Explanation	[[Explanation::The patient has primary syphilis, which is characterized by the appearance of a painless chancre (an ulcer-like lesion). Syphilis is caused by infection with the spirochete Treponema pallidum, and can be identified by darkfield microscopy or fluorescent antibody staining. The fluorescent Treponemal Antibody Absorption (FTA-Abs) assay uses treponemal antigens fixed to a glass slide to detect whether antibodies specific to the organism are present in the serum from a human sample. The FTA-Abs assay is more specific than the rapid plasma reagent (RPR) or VDRL tests for syphilis. While FTA-Abs may be used to confirm a presumed diagnosis of syphilis, RPR and VDRL are sensitive, but unspecific, tests that may be used for screening purposes. None of the other answer choices could be used to visualize Treponema. Educational Objective: Primary syphilis, characterized by the presence of a self-resolving, painless genital chancre, is caused by Treponema pallidum. Syphilis may be diagnosed using darkfield microscopy or Fluorescent Treponemal Antibody Absorption (FTA-Abs) assay. References: First Aid 2014 page 122 (stains) First Aid 2014 page 141 (Syphilis)]]
Approved	Approved::Yes
Keyword	WBRKeyword::Microbiology, WBRKeyword::Syphilis, WBRKeyword::Bacteria, WBRKeyword::Laboratory, WBRKeyword::Sex, WBRKeyword::STD, WBRKeyword::STI, WBRKeyword::Sexually transmitted
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