Question:medium
A truck driver presented with a painless, demarcated ulcer on penis with inguinal lymphadenopathy. What is the best method of visualization of motility of the probable causative agent?
A truck driver presented with a painless, demarcated ulcer on penis with inguinal lymphadenopathy. What is the best method of visualization of motility of the probable causative agent?
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The causative agent of a painless penile chancre is a spirochete best visualized alive by a special microscopy technique that makes it appear bright on a dark background.
Updated On: Jun 23, 2026
- Dark field microscopy
- Light microscopy
- Fluorescent microscopy
- Electron microscopy
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The Correct Option is A
Solution and Explanation
Systematic reasoning:
Clinical clues: painless penile ulcer + inguinal lymphadenopathy = primary syphilis caused by $\text{Treponema pallidum}$.
Why dark field microscopy?
- $T. pallidum$ is 6--20 microns long but only 0.1--0.18 microns wide -- below the resolution limit of standard light microscopy
- It does not stain with Gram or Giemsa reliably
- Cannot be cultured on artificial media
- In dark field microscopy, oblique light makes the thin spirochetes appear bright white against a dark background, allowing direct visualization of their characteristic corkscrew motility
Other direct detection methods include DFA-TP (direct fluorescent antibody) and NAATs -- but for motility specifically, dark field is required.
\[\boxed{\text{Dark field microscopy}}\]
Clinical clues: painless penile ulcer + inguinal lymphadenopathy = primary syphilis caused by $\text{Treponema pallidum}$.
Why dark field microscopy?
- $T. pallidum$ is 6--20 microns long but only 0.1--0.18 microns wide -- below the resolution limit of standard light microscopy
- It does not stain with Gram or Giemsa reliably
- Cannot be cultured on artificial media
- In dark field microscopy, oblique light makes the thin spirochetes appear bright white against a dark background, allowing direct visualization of their characteristic corkscrew motility
Other direct detection methods include DFA-TP (direct fluorescent antibody) and NAATs -- but for motility specifically, dark field is required.
\[\boxed{\text{Dark field microscopy}}\]
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