A 10-year old child presents in your office with unilateral follicular conjunctivitis and ipsilateral adenopathy. What is the most likely cause?

The presentation of unilateral follicular conjunctivitis combined with ipsilateral adenopathy is characteristic of cat-scratch disease, which is caused by the bacterium Bartonella henselae. This disease commonly occurs in children and is typically associated with a prior feline scratch or bite. The conjunctivitis results from the infectious process, and the associated adenopathy reflects the systemic response to the infection, making this a very likely diagnosis in a child with these symptoms.

In contrast, measles is typically associated with a more generalized rash and systemic symptoms, including fever and respiratory involvement, rather than isolated conjunctivitis and adenopathy. Coccidioidomycosis and toxoplasmosis are both more systemic infections and usually present with different ocular signs and symptoms, such as chorioretinitis or other more diffuse manifestations rather than localized conjunctivitis and adenopathy. Thus, the combination of symptoms presented in this case strongly indicates cat-scratch disease as the most plausible cause.