USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 70-year-old man comes into the clinic due pain when he wears his dentures, which has developed over the last week. Physical examination shows erythema on his gums and white plaque on the buccal mucosa. Microscopic analysis of the plaque shows yeast-like cells and pseudohyphae similar to those shown below.
Image 1: Micrograph of cells from the white plaque
Spores from the sample are suspended in animal serum and left to grow for two hours at 37°C (98.6°F). Which of the following is most likely to be seen in the tube?
Candida is a yeast, not the maple syrup-loving country in North America - although Candida can be found in Canada as well!
Candida sometimes causes a mild yeast infection, but in some situations, can get into the bloodstream and cause severe illness.
Now, there are various types of Candida species, and over twenty of them cause disease in humans - C. albicans, C. parapsilosis, C. tropicalis, C. glabrata, C. krusei, C. auris, the list goes on.
Of these, the most common one is C. albicans. Candida is found throughout the body; it likes warm, moist environments like the mouth, the diaper region of babies, and in women it can be found in the vagina.
Now, it’s normal for microbes - bacteria, fungi, and viruses - to live all over the body, but each microbe is slightly different in terms of whether it’s colonizing the body - in other words just living and not causing any problems, or whether it’s infecting the body, causing some degree of tissue damage or destruction.
An important factor is exactly how much of a microbe is present.
Candida is considered an opportunistic microbe.
When the amount of Candida is relatively low, it's harmless.
But if a person’s immune system is weakened or if there’s less competition for the Candida, then the amount of Candida can increase - and that’s called Candida overgrowth.
Now, Candida can exist in multiple forms - it’s a bit like a chameleon.
Sometimes the cells can appear round or oval and these are called yeast cells, other times it can appear like hyphae where it looks like long thin filaments - kind of like a segmented cactus plant.
It can also take an in-between appearance called pseudohyphae.
Each of these morphologies or “looks” reflect the same Candida cells that are expressing different protein profiles, and they give the cells different properties.
When the Candida is in “yeast mode” it’s better at moving from one part of the body to another, whereas when it’s in “filamentous mode” it’s better at invading tissues.
Candida typically lives on the skin or mucous membranes, and when it starts to overgrow it can damage nearby tissue.
There are a few patterns of injury, the most common one is pseudomembranous candidiasis, and it’s primarily due to a weakened immune system that allows for Candida overgrowth.
The result is destruction of the stratified squamous epithelium layer, which is the outermost layer of the skin or mucous membranes.
This causes accumulation of the destroyed cells and the keratin protein that fills that outermost layer, forming a white lesion, called a pseudomembrane, that looks kind of like “cottage cheese”.
The white lesions aren’t typically painful and they can be scraped away with a tongue depressor, leaving behind a red mucosal base which sometimes bleeds.
Since the underlying cause is a weakened immune system, it’s fairly common in young infants and the elderly, both groups that have a relatively weak immune system.
It can also be related to an immunosuppressive condition like diabetes or HIV, or from immunosuppressive medical treatments like steroids - including inhaled steroids, as well as radiotherapy or chemotherapy.
Another pattern of injury is called erythematous candidiasis and that typically results from a change in the levels of microbial competition keeping Candida in check.
For example, a course of antibiotics or tobacco smoking, which both selectively destroy certain bacterial populations more than they affect Candida.
The opposite is true as well, sometimes there are mechanical devices like braces that favor Candida growth more than the growth of other microbes.
In either situation, the result is an overgrowth of Candida which causes increased blood flow to the affected tissue with red painful lesions.
In a lot of situations there’s a mixed pattern of injury with both a pseudomembranous and erythematous component.