A 9-year-old presents with fever and a pruritic rash after starting penicillin. What is the most likely diagnosis?

The scenario presents a 9-year-old who developed a fever and a pruritic rash after beginning penicillin treatment. The key features here are the onset of symptoms after medication administration and the combination of fever with a rash, which are indicative of a systemic reaction rather than a localized skin response.

A serum sickness-like reaction can occur with certain medications, including penicillin. This type of reaction is characterized by fever, rash, and arthralgia, and occurs due to the formation of immune complexes in response to the drug. It typically appears one to two weeks after exposure to the medication, aligning with the timeline for the child's symptoms.

In contrast, contact dermatitis generally arises from direct skin exposure to an irritant or allergen and would not typically present with systemic symptoms like fever. Drug-related arthritis may cause joint inflammation but does not usually present with a fever and pruritic rash. Allergic urticaria is more localized and might not be associated with fever and systemic symptoms to the same extent as a serum sickness-like reaction.

Therefore, given the systemic nature of the child's symptoms following the initiation of penicillin, a serum sickness-like reaction is the most plausible diagnosis.