WBR0047
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| Author | [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]] |
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| 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. Which of the following tests is the most helpful in diagnosing this patient's condition?
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| Answer A | AnswerA::Giemsa stain |
| Answer A Explanation | [[AnswerAExp::Giemsa stain is used to identify Borrelia spp., Plasmodium spp., and Chlamydia spp.]] |
| 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 causes pneumonia in individuals with advanced HIV infection.]] |
| Answer D | AnswerD::Fluorescent antibody |
| Answer D Explanation | [[AnswerDExp::Treponema pallidum can optimally be visualized 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 diagnose syphilis.]] |
| Answer E | AnswerE::India ink |
| Answer E Explanation | [[AnswerEExp::India ink stain is used to visualize Cryptococcus neoformans. Cryptococcus neoformans is associated with meningitis in HIV-positive individuals.]] |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::Primary syphilis is characterized by the appearance of a painless chancre (an ulcer-like lesion). It is caused by an 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 patient's serum. The FTA-Abs assay is more specific than the rapid plasma reagin (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 tests in the other answer choices could be used to diagnose or visualize Treponema pallidum. 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. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Microbiology, WBRKeyword::Syphilis, WBRKeyword::Bacteria, WBRKeyword::Laboratory, WBRKeyword::Sex, WBRKeyword::STD, WBRKeyword::STI, WBRKeyword::Sexually transmitted |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
