What organism is likely identified in an elderly woman with suspected cat-scratch disease, characterized by a small, curved gram-negative bacillus?

Bartonella spp. is the correct identification in the case presented, particularly due to its association with cat-scratch disease (CSD), which is primarily caused by Bartonella henselae. This organism is characterized as a small, curved gram-negative bacillus, aligning perfectly with the description given in the question.

Cat-scratch disease typically arises from a scratch or bite from a cat, and the clinical manifestations often include lymphadenopathy, fever, and fatigue, especially in vulnerable populations like the elderly. The biological and epidemiological profile of Bartonella supports its identification in this context.

Other organisms listed do not fit the clinical picture or morphological characteristics associated with CSD. For instance, Brucella spp. is more commonly linked to zoonotic infections from livestock, while Kingella spp. and Haemophilus spp. are not primarily associated with cat-related infections or the symptoms typical of cat-scratch disease. Thus, the association of Bartonella spp. with the condition, along with its specific morphological traits, makes it the most probable organism in this scenario.