**Core Concept**
Infectious mononucleosis, also known as glandular fever, is a disease caused by a viral infection that primarily affects the lymphoid system. The virus responsible for this condition is a member of the herpesvirus family.

**Why the Correct Answer is Right**
The correct answer is the Epstein-Barr virus (EBV), a type of herpesvirus that primarily infects B lymphocytes. EBV is transmitted through saliva and other bodily fluids, leading to the proliferation of infected B cells, which in turn causes the symptoms associated with infectious mononucleosis. The virus is highly contagious and can cause a range of symptoms, from mild to severe, including fever, sore throat, swollen lymph nodes, and fatigue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the virus responsible for infectious mononucleosis is not the Cytomegalovirus (CMV), although CMV can also infect B cells and cause similar symptoms.
**Option B:** This option is incorrect because the Human Immunodeficiency Virus (HIV) is not responsible for infectious mononucleosis, although HIV can target and destroy B cells.
**Option C:** This option is incorrect because the Varicella-Zoster virus is responsible for chickenpox and shingles, not infectious mononucleosis.

**Clinical Pearl / High-Yield Fact**
Infectious mononucleosis is often referred to as the "kissing disease" due to its transmission through saliva. EBV can also cause other conditions, such as Burkitt's lymphoma and nasopharyngeal carcinoma.

**Correct Answer: D. Epstein-Barr virus.**