Question Dissection

Question:

A mailman gets a severe bite wound from a pit bull guarding a junkyard. The wound is cleansed and he receives a booster injection of tetanus toxoid and an injection of penicillin G. Several days later, the wound is inflamed and purulent. The exudate is cultured on blood agar and yields gram-negative rods. Antibiotic sensitivity tests are pending. The most likely agent to be isolated is

A. Actinomyces israelii

B. Leptospira interrogans

C. Clostridium tetani

D. Pasteurella multocida

E. Capnocytophagia canimorsus





Answer: (D) Pasteurella multicida

It is important to know the infections you get from a cat/dog bite, scratch, and urine.

Bite: Pasteurella multicida
Scratch: Bartonella henselae
Urine: Leptospira interrogans

Of the above choices, only two are gram negative rods, Pasteurella and Capnocytophagia. Actinomyces is "actin like a myces" so it is a gram positive filamentous rod. Leptospira is a spirochete. Clostridium is a gram positive rod. The tricky one is (E) Capnocytophagia canimorsus. If you look at the name, Capnocytophagia, you can see the "capno" and the "cytophagia." "Capno" refers to carbon dioxide and "cytophaga" means "cell eater." Capnocytophagia is a fastidious grower which requires a microaerophilic environment to grow so it probably wouldn't grow on the above blood agar.

One of the most my important resources is my Stedman's medical dictionary. I have it on my Blackberry, computer, and Palm. Stedman's is nice because it gives you the Latin and Greek roots of the words. How can you answer a question if you don't know the difference between sclerosis and stenosis? In the question above, knowing that "capno" meant carbon dioxide, gave me enough information to get a similar question right on my USMLE Step 1.