## **Core Concept**
The clinical presentation described suggests a diagnosis of **Infectious Mononucleosis (IMN)**, also known as glandular fever, which is most commonly caused by **Epstein-Barr Virus (EBV)**. This condition is characterized by fever, sore throat, lymphadenopathy, hepatosplenomegaly, and the presence of atypical lymphocytes in the peripheral blood.

## **Why the Correct Answer is Right**
The correct answer involves the receptor used by **Epstein-Barr Virus (EBV)** to enter the host cells. EBV primarily infects **B lymphocytes**, and the receptor involved in this process is **CD21**, also known as **CR2 (Complement Receptor 2)**. EBV uses its envelope glycoprotein **gp350/220** to bind to CD21 on B cells, facilitating viral entry.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not correspond to the receptor used by EBV for entering host cells.
- **Option B:** Similarly, this option does not accurately represent the receptor involved in EBV pathogenesis.
- **Option D:** This is also incorrect as it does not correctly identify the receptor used by EBV.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of **atypical lymphocytes** (also known as Downey cells) in the peripheral blood smear is a hallmark of infectious mononucleosis. These cells are a type of activated lymphocyte. The **Monospot test** or **Heterophile antibody test** is a commonly used serological test for the diagnosis of EBV infection, although it is not specific for EBV and can yield false positives.

## **Correct Answer Line**
**Correct Answer: C. CD21.**