Question

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. Dark field microscopy
• B. Light microscopy
• C. Fluorescent microscopy
• D. Electron microscopy

Hint:

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.

Answer 1

The Correct Option is A

Step 1: Diagnose the clinical picture. A painless, demarcated (punched-out) ulcer on the penis with inguinal lymphadenopathy is the classic presentation of primary syphilis (a chancre), caused by Treponema pallidum.

Step 2: Recall properties of Treponema pallidum. T. pallidum is a spirochete that is too thin (0.1--0.18 microns wide) to be visible under ordinary light microscopy. It cannot be cultured on artificial media and does not take up Gram stain.

Step 3: Identify the correct microscopy method. Dark field microscopy (DFM) is the gold standard for direct visualization of live T. pallidum, demonstrating its characteristic corkscrew motility (rotation and flexion). In DFM, the specimen is illuminated from the side so organisms appear bright against a dark background.

Step 4: Eliminate other options. Light microscopy cannot resolve the organism. Fluorescent microscopy (DFA-TP) can detect antigen but is not primarily used to see motility. Electron microscopy shows ultrastructure but is not used for motility assessment.

Answer: Dark field microscopy