**Core Concept**
The patient's presentation is suggestive of Infectious Mononucleosis (IM), a viral illness caused by Epstein-Barr Virus (EBV). The symptoms include sore throat, fever, malaise, lymphadenopathy, splenomegaly, and petechiae, which are characteristic of this condition.

**Why the Correct Answer is Right**
Infectious Mononucleosis is diagnosed primarily through clinical evaluation, but laboratory tests can aid in confirmation. The correct answer is a test that detects the presence of heterophile antibodies, which are a hallmark of IM. These antibodies are produced in response to EBV infection and can be detected using the Monospot test. This test detects the presence of heterophile antibodies in the patient's serum, which react with sheep red blood cells. A positive result is diagnostic of IM.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood culture may be useful in identifying bacterial infections that can cause similar symptoms, but it is not the most specific test for IM. Blood cultures are more relevant in the context of suspected sepsis or bacterial infections.

**Option B:** Complete Blood Count (CBC) can reveal anemia, thrombocytopenia, and leukocytosis, which are common in IM. However, these findings are non-specific and can be seen in many other conditions.

**Option C:** Liver function tests (LFTs) may be elevated in IM due to EBV-induced hepatitis. However, LFTs are not specific for IM and can be elevated in many other conditions.

**Clinical Pearl / High-Yield Fact**
The Monospot test is a rapid test that detects heterophile antibodies in the patient's serum. A positive result indicates Infectious Mononucleosis, but a negative result does not rule out the diagnosis. Clinical correlation and additional laboratory tests, such as EBV serology, may be necessary to confirm the diagnosis.

**Correct Answer:** C.