A 35-year-old woman presented with swelling and pain of her left wrist of ten days' duration. Twelve days ago she was bitten on the left hand by her pet cat. Two days later she developed pain, redness, and swelling of her hand. O/E, Mild fever Erythematous left wrist and thenar eminence Reduced range of motion of fingers The white blood cell count was 13,000/ml, with a marked increase in the percentage of immature granulocytes. Gram stain of the wrist fluid was done. What is the most likely cause?

The correct answer is A (Pasteurella multocida), a gram-negative rod that frequents the mouths of some animals, paicularly cats, but also dogs. The extreme sharpness of feline teeth causes enormous pressure at the site of the puncture during a bite, allowing inoculation of the organism deep into tissues. B (Baonella henselae) is incorrect, although it is a gram-negative rod associated with exposure to cats. In immunocompetent hosts, the principal condition caused by this organism, which is carried mainly on the paws of cats, is cat scratch disease, whose name implies transmission by cat scratches not cat bites. C (Streptobacillus moniliformis) is also a gram-negative rod, pleomorphic, which is most commonly acquired by the bite or scratch of rats or mice. Although it may be carried--and transmitted--by carnivores that prey on these rodents, it is less characteristically associated with cat induced injuries than with rat or mouse exposure. The simplest reason that D (Lactobacillus casei) cannot be a correct answer because it is a gram-positive rod, not a gram-negative one.