**Core Concept**
Infectious mononucleosis, also known as glandular fever, is a viral illness that primarily affects young adults and is caused by the Epstein-Barr virus (EBV), a member of the herpesvirus family. EBV is a double-stranded DNA virus that infects B lymphocytes, leading to various systemic symptoms and clinical manifestations.

**Why the Correct Answer is Right**
The Epstein-Barr virus is the primary causative agent of infectious mononucleosis, accounting for approximately 90% of cases worldwide. EBV infection occurs through saliva and other bodily fluids, often transmitted through close contact with an infected individual. The virus then infects B lymphocytes, leading to the proliferation of these cells and the subsequent production of heterophil antibodies, which are responsible for the positive Paul-Bunnell test, a diagnostic hallmark of the disease. The infection can lead to symptoms such as fever, sore throat, fatigue, and lymphadenopathy.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect as infectious mononucleosis is most prevalent in young adults, particularly those between 15 and 24 years old, rather than children less than 14 years old.
**Option B:** This is incorrect as infectious mononucleosis is caused by the Epstein-Barr virus (EBV), not a rhabdovirus, which is a family of viruses that includes rabies and vesicular stomatitis.
**Option D:** While affected persons do respond to treatment with the production of heterophil antibodies, this is not a defining characteristic of the disease, and the statement does not accurately convey the primary cause of infectious mononucleosis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that infectious mononucleosis can be a significant cause of morbidity, particularly in young adults, and can lead to complications such as splenic rupture, thrombocytopenia, and neurological manifestations. The diagnosis is primarily based on clinical presentation, laboratory findings, and the presence of heterophil antibodies.

**✓ Correct Answer: C. The causative pathogen is an Epstein-Barr virus**