Which microorganism is most clearly associated with an exogenous infection (acquired from outside the host rather than from the host’s normal microbiota)?

• A. Escherichia coli from the patient’s own intestinal microbiota causing a urinary tract infection
• B. Candida albicans from the patient’s normal vaginal microbiota causing vulvovaginal candidiasis
• C. Staphylococcus epidermidis from the patient’s skin flora causing infection of an indwelling catheter
• D. Influenza A virus acquired by inhaling respiratory droplets from an infected person

Answer: (D)

Verified step by step guidance

1. Understand the definition of an exogenous infection: it is an infection caused by a microorganism that originates from outside the host, rather than from the host's own normal microbiota.
2. Review each microorganism and its source: Escherichia coli causing a urinary tract infection typically comes from the patient's own intestinal microbiota, so this is an endogenous infection.
3. Candida albicans causing vulvovaginal candidiasis is also from the patient's normal vaginal microbiota, making it an endogenous infection as well.
4. Staphylococcus epidermidis causing infection of an indwelling catheter usually originates from the patient's skin flora, which is part of the normal microbiota, so this is also endogenous.
5. Influenza A virus, however, is acquired from the environment, specifically by inhaling respiratory droplets from an infected person, making it an exogenous infection.