**Core Concept**
The Paul-Bunnell test, also known as the heterophile antibody test, is used to diagnose infectious mononucleosis caused by the Epstein-Barr virus (EBV). This test detects the presence of heterophile antibodies in the patient's serum, which are a hallmark of EBV infection.

**Why the Correct Answer is Right**
The Paul-Bunnell test works by detecting the presence of heterophile antibodies, which are IgM antibodies that react with the red blood cells of sheep or horse. These antibodies are produced in response to the EBV infection and are a specific indicator of the disease. The test involves mixing the patient's serum with sheep red blood cells and observing for agglutination, which indicates the presence of heterophile antibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Paul-Bunnell test is not used to diagnose toxoplasmosis, which is a different parasitic infection.
**Option B:** This option is incorrect because the Weil-Felix test is used to diagnose rickettsial infections, not infectious mononucleosis.
**Option C:** This option is incorrect because the VDRL test is used to diagnose syphilis, which is a different bacterial infection.

**Clinical Pearl / High-Yield Fact**
The Paul-Bunnell test is a classic example of a serological test that detects the presence of heterophile antibodies, which are a hallmark of EBV infection. This test is still widely used today to diagnose infectious mononucleosis, although more modern tests such as the EBV-specific antibody test are also available.

**Correct Answer:** D. Infectious mononucleosis