Communicable diseases: early syphilis diagnosis: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Communicable diseases: early syphilis diagnosis

Sample item

A man presents with a single painless, indurated genital ulcer of 10 days with non-tender inguinal lymph nodes. Serological tests for syphilis are still non-reactive. Which test can confirm the diagnosis at this stage?

AVenereal Disease Research Laboratory testBGram stain of the ulcer scrapingCDark-field microscopy of ulcer exudateDTreponema pallidum haemagglutination assay

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Dark-field microscopy of ulcer exudate

In primary syphilis, antibodies may not yet be detectable, so serology can be negative in the first weeks. Dark-field microscopy of fluid from the chancre shows motile spirochaetes and gives an immediate diagnosis. Serology becomes reactive over the following weeks.

Option by option

A. Venereal Disease Research Laboratory test
VDRL is a non-treponemal antibody test and is often negative early in primary syphilis.
B. Gram stain of the ulcer scraping
The spirochaete is too thin to be seen on Gram stain.
C. Dark-field microscopy of ulcer exudate (correct)
Correct. Dark-field microscopy shows Treponema pallidum directly while serology is still negative.
D. Treponema pallidum haemagglutination assay
TPHA is a treponemal antibody test and is also commonly negative this early.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, Dark-field microscopy of ulcer exudate. In primary syphilis, antibodies may not yet be detectable, so serology can be negative in the first weeks.

Why is "Venereal Disease Research Laboratory test" not the answer?

VDRL is a non-treponemal antibody test and is often negative early in primary syphilis.

Why is "Gram stain of the ulcer scraping" not the answer?

The spirochaete is too thin to be seen on Gram stain.

Why is "Treponema pallidum haemagglutination assay" not the answer?

TPHA is a treponemal antibody test and is also commonly negative this early.

What concept does this question test?

It tests Communicable diseases: early syphilis diagnosis in PSM. Dark-field microscopy of fluid from the chancre shows motile spirochaetes and gives an immediate diagnosis. Serology becomes reactive over the following weeks.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Communicable diseases: early syphilis diagnosis?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.