## **Core Concept**
The question presents a clinical scenario involving a 6-year-old child with symptoms of hepatosplenomegaly, generalized lymphadenopathy, fever, and a rash that developed after administration of ampicillin. This combination of symptoms suggests an infectious or hematological condition. The key detail here is the development of a rash after ampicillin, which is a classic clue.

## **Why the Correct Answer is Right**
The correct answer, **C. Infectious Mononucleosis (IMN)**, also known as glandular fever, is caused by Epstein-Barr Virus (EBV). IMN typically presents with fever, sore throat, hepatosplenomegaly, lymphadenopathy, and fatigue. A distinctive feature of IMN is that patients often develop a maculopapular rash after being treated with ampicillin or other penicillins, in about 80-100% of cases. This occurs due to the immune system's altered response in the presence of EBV infection.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While systemic lupus erythematosus (SLE) can present with a wide range of symptoms including lymphadenopathy and hepatosplenomegaly, the development of a rash after ampicillin is not a characteristic feature.
- **Option B:** Toxoplasmosis can cause lymphadenopathy and sometimes hepatosplenomegaly but does not characteristically cause a rash after antibiotic administration.
- **Option D:** Cytomegalovirus (CMV) infection can present similarly to EBV with fever, lymphadenopathy, and hepatosplenomegaly but is less commonly associated with a rash after ampicillin.

## **Clinical Pearl / High-Yield Fact**
A crucial clinical pearl is that the appearance of a rash after administering ampicillin to a patient suspected of having a fever and lymphadenopathy is highly suggestive of **Infectious Mononucleosis**. This association is a classic example of a drug reaction that is indicative of the underlying diagnosis rather than an allergy to the drug itself.

## **Correct Answer: C. Infectious Mononucleosis.**