**Core Concept**
The patient's symptoms suggest an infection, likely caused by the **Epstein-Barr virus (EBV)**, which is known to cause **infectious mononucleosis**. This condition is characterized by **lymphadenopathy**, **sore throat**, and the presence of **atypical lymphocytes** on a blood film.

**Why the Correct Answer is Right**
The presence of a positive **heterophil antibody test**, also known as the **Monospot test**, is a key diagnostic indicator of **infectious mononucleosis**. This test detects the presence of **heterophil antibodies**, which are a type of antibody that reacts with **sheep red blood cells**. The combination of clinical symptoms and a positive heterophil antibody test points towards **infectious mononucleosis** as the most likely diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not match the description of the correct answer.
**Option B:** Incorrect because it is not related to the clinical presentation.
**Option C:** Incorrect as it is not associated with a positive heterophil antibody test.
**Option D:** Incorrect because it does not align with the symptoms described.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **infectious mononucleosis** caused by **EBV** can lead to **splenomegaly** and, in rare cases, **splenic rupture**.

**Correct Answer:** D. Infectious Mononucleosis.