**Core Concept**
Infectious mononucleosis, commonly referred to as "kissing disease," is a clinical syndrome caused by primary Epstein-Barr virus (EBV) infection. EBV is a member of the herpesvirus family and is transmitted through saliva, leading to a self-limiting illness characterized by fever, sore throat, lymphadenopathy, and hepatosplenomegaly.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of infectious mononucleosis. EBV infects B lymphocytes, leading to a polyclonal activation of these cells, which in turn results in the production of atypical lymphocytes. This lymphocytosis is a hallmark of the disease and is responsible for the clinical manifestations of fever, sore throat, and lymphadenopathy. Additionally, EBV-infected hepatocytes can lead to hepatosplenomegaly.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because infectious mononucleosis is not caused by cytomegalovirus (CMV), which is another member of the herpesvirus family but is associated with a different clinical syndrome.
**Option B:** This option is incorrect because human immunodeficiency virus (HIV) infection is not typically associated with the clinical manifestations of infectious mononucleosis, although EBV can cause lymphoproliferative disorders in HIV-positive individuals.
**Option C:** This option is incorrect because toxoplasmosis is a different parasitic infection that can cause a range of clinical symptoms, including lymphadenopathy, but is not associated with the clinical syndrome of infectious mononucleosis.
**Option D:** This option is incorrect because acute HIV infection can present with a flu-like illness, but it does not typically cause the clinical manifestations of infectious mononucleosis.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that EBV infection is a common cause of infectious mononucleosis, and a diagnosis can be confirmed by detecting specific antibodies against EBV in the serum. A classic clinical pearl is that the presence of atypical lymphocytes on a peripheral smear is a strong indicator of EBV infection.

**Correct Answer:** A. Infectious mononucleosis is caused by Epstein-Barr virus (EBV) infection.