A 7-year-old girl with painful swelling under her arm, showing pleomorphic bacilli in biopsy. What is the causal organism?

The presence of painful swelling in the underarm area, along with the identification of pleomorphic bacilli in the biopsy, strongly suggests an infection associated with Bartonella henselae. This organism is known to be the causative agent of cat scratch fever, which can lead to lymphadenopathy and localized swelling typically seen in children after exposure to cats.

Bartonella henselae is characterized by its pleomorphic nature, which means it can exist in various forms and sizes under microscopic examination. The clinical presentation in this case aligns well with the typical symptoms of a cat scratch fever infection, often presenting as a painful lymphadenopathy in young patients.

Other potential organisms listed, while significant in other contexts, do not fit as neatly with the symptoms and biopsy findings presented. For instance, Brucella melitensis is typically associated with undulant fever and exposure to infected livestock, while Burkholderia mallei is linked to glanders disease, and Francisella tularensis is associated with tularemia. These infections generally do not present with the specific pleomorphic bacilli findings described in this case.

Therefore, the identification of pleomorphic bacilli in the context of the clinical symptoms strongly supports Bartonella henselae as the causal