A 30-year-old man develops a pustular lesion at the site of a cat scratch on his forearm. This is followed 1 week later by malaise, fever, and lymphadenopathy. On examination, he has tender axillary lymph nodes. Which of the following is the most likely causative organism?
**Core Concept**
Cat scratch disease (CSD) is a zoonotic infection caused by the bacterium Bartonella henselae, typically transmitted through scratches or bites from infected cats. This bacterium targets the reticuloendothelial system, leading to the characteristic symptoms of lymphadenopathy, fever, and malaise.
**Why the Correct Answer is Right**
The correct answer is Bartonella henselae. This bacterium is a gram-negative rod that invades and proliferates within endothelial cells and histiocytes, leading to the formation of granulomas and lymphadenopathy. The bacteria are transmitted to humans through the saliva of infected cats, often through scratches or bites. The incubation period of CSD typically ranges from 3 to 10 days, with the onset of symptoms usually occurring 5-7 days after exposure.
**Why Each Wrong Option is Incorrect**
**Option A:** Pasteurella multocida is a bacterium commonly found in the mouths of cats and dogs, but it is not the primary cause of cat scratch disease. Pasteurella multocida can cause infection through animal bites, but it is not the organism responsible for the characteristic lymphadenopathy seen in CSD.
**Option B:** Capnocytophaga canimorsus is another bacterium that can be found in the mouths of cats and dogs, but it is not the primary cause of cat scratch disease. Capnocytophaga canimorsus can cause severe infections in immunocompromised individuals, but it is not associated with the characteristic symptoms of CSD.
**Option C:** Staphylococcus aureus is a common cause of skin and soft tissue infections, but it is not the primary cause of cat scratch disease. Staphylococcus aureus can cause a range of symptoms, from mild skin infections to life-threatening conditions, but it is not associated with the characteristic lymphadenopathy seen in CSD.
**Clinical Pearl / High-Yield Fact**
The characteristic "papule" or "pustule" at the site of the cat scratch, followed by lymphadenopathy and systemic symptoms, is a classic presentation of cat scratch disease. This clinical picture should prompt consideration of Bartonella henselae as the causative organism.
**Correct Answer:** .