**Core Concept**
Infectious mononucleosis, commonly known as "kissing disease," is a clinical syndrome caused by Epstein-Barr virus (EBV) infection. EBV is a member of the herpesvirus family and primarily spreads through saliva, leading to its association with close contact.

**Why the Correct Answer is Right**
Infectious mononucleosis is characterized by fever, sore throat, fatigue, and lymphadenopathy. The clinical presentation is often accompanied by atypical lymphocytosis, which is a hallmark of EBV infection. The immune response to EBV involves the activation of CD8+ T cells, which play a crucial role in controlling the virus replication.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because infectious mononucleosis is not caused by cytomegalovirus (CMV), although CMV can also cause mononucleosis-like symptoms in immunocompromised individuals.
**Option B:** This option is incorrect because toxoplasmosis is a parasitic infection caused by Toxoplasma gondii, which has a different clinical presentation and pathophysiology compared to infectious mononucleosis.
**Option C:** This option is incorrect because coxsackievirus is a type of enterovirus that causes a variety of clinical syndromes, including hand-foot-and-mouth disease and aseptic meningitis, but it is not associated with infectious mononucleosis.

**Clinical Pearl / High-Yield Fact**
Infectious mononucleosis is often confused with other viral infections, such as CMV or adenovirus, but the presence of atypical lymphocytosis and a positive heterophile antibody test (Monospot test) can help distinguish it from other conditions.

**Correct Answer:** C.