## **Core Concept**
The patient presents with symptoms suggestive of infectious mononucleosis (IMN), also known as glandular fever, which is primarily caused by Epstein-Barr virus (EBV). Key features include sore throat, fever, malaise, lymphadenopathy, hepatosplenomegaly, and atypical lymphocytosis on the blood smear.

## **Why the Correct Answer is Right**
The clinical presentation and laboratory findings, such as atypical lymphocytosis, elevated liver enzymes (AST, ALT), and elevated LDH, support the diagnosis of infectious mononucleosis. The next best investigation modality to confirm the diagnosis would be the **Monospot test (or rapid EBV test)**, which detects the presence of heterophile antibodies, a hallmark of acute EBV infection. This test is quick and can support the clinical diagnosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, other blood tests like complete blood count (CBC) have already been done and show atypical lymphocytosis.
- **Option B:** Imaging studies like ultrasound or CT scans could assess spleen and liver size but are not diagnostic for EBV infection.
- **Option C:** A throat culture has already been done and is negative for group A streptococci, which can sometimes present similarly but does not help in diagnosing EBV.
- **Option D:** This option is not provided.

## **Clinical Pearl / High-Yield Fact**
Infectious mononucleosis should be suspected in patients with prolonged fever, sore throat, lymphadenopathy, and hepatosplenomegaly. A key clinical pearl is that the **Monospot test** can be negative early in the disease course, and it may take several weeks for heterophile antibodies to become detectable.

## **Correct Answer:** .