## **Core Concept**
Heterophile-negative mononucleosis refers to a condition that presents with symptoms similar to infectious mononucleosis (caused by Epstein-Barr virus, EBV) but lacks the typical heterophile antibodies (also known as Paul-Bunnell test) and negative EBV titers. This condition can be caused by various viruses.

## **Why the Correct Answer is Right**
Toxoplasmosis, caused by *Toxoplasma gondii*, is a known cause of heterophile-negative mononucleosis. It presents with lymphadenopathy, fever, and sometimes lymphocytosis, mimicking infectious mononucleosis but with negative EBV titers and heterophile antibody tests. The diagnosis is confirmed by serological tests for *T. gondii*.

## **Why Each Wrong Option is Incorrect**
* **Option A:** While certain drugs and conditions can cause a mononucleosis-like syndrome, they are not typically classified under infectious causes of heterophile-negative mononucleosis.
* **Option B:** Cytomegalovirus (CMV) can cause a similar syndrome but is less commonly associated with the term "heterophile-negative mononucleosis" in comparison to Toxoplasmosis.
* **Option D:** Human herpesvirus 6 (HHV-6) and other viruses can cause febrile illnesses but are not as commonly linked to this specific clinical presentation.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is to consider toxoplasmosis in the differential diagnosis of patients presenting with lymphadenopathy and a mononucleosis-like syndrome but with negative EBV and heterophile antibody tests, especially if there's exposure to cats or undercooked meat.

## **Correct Answer:** C. Toxoplasmosis.