Which of the following is seen in dark field microscopy: September 2009
**Core Concept**
Dark field microscopy is a technique used to visualize microorganisms, particularly in the context of syphilis diagnosis. It involves illuminating the sample with a special light that makes the microorganisms appear bright against a dark background.
**Why the Correct Answer is Right**
In dark field microscopy, the microorganism *Treponema pallidum* appears as a spirochete with a characteristic corkscrew shape. This is due to the unique morphology of the organism, which has a spiral body and endoflagella that allow it to move in a corkscrew pattern. The dark field microscopy technique highlights the movement and morphology of these microorganisms, making them visible against the dark background.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because dark field microscopy is not typically used to visualize *Candida* species, which are usually seen on wet mounts or Gram stains.
* **Option B:** This option is incorrect because *Staphylococcus aureus* is a gram-positive cocci that is typically visualized on Gram stains or methylene blue stains, not in dark field microscopy.
* **Option C:** This option is incorrect because *Haemophilus influenzae* is a gram-negative bacillus that is typically visualized on Gram stains or on chocolate agar plates, not in dark field microscopy.
**Clinical Pearl / High-Yield Fact**
Dark field microscopy is a valuable diagnostic tool in the early stages of syphilis, as it can detect *Treponema pallidum* in the cerebrospinal fluid (CSF) of patients with neurosyphilis.
**Correct Answer: C. Treponema pallidum**