A patient with sore throat has a positive Paul Bunnell test. The causative organism is:
**Core Concept**
The Paul Bunnell test, also known as the heterophile antibody test, is a serological diagnostic tool used to detect the presence of heterophile antibodies in patients suspected of having infectious mononucleosis (IMN), a disease caused by the Epstein-Barr virus (EBV).
**Why the Correct Answer is Right**
The Paul Bunnell test detects the presence of heterophile antibodies, which are IgM antibodies that react with sheep erythrocytes. These antibodies are produced in response to the EBV infection and are a hallmark of IMN. The test is based on the principle of agglutination, where the heterophile antibodies cause the sheep erythrocytes to clump together, indicating a positive result. The EBV infection triggers the activation of B lymphocytes, which produce these antibodies.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Paul Bunnell test is specific to EBV infection and not to other viral infections such as CMV or HIV.
**Option B:** This option is incorrect because the Paul Bunnell test is not used to diagnose bacterial infections such as streptococcal pharyngitis.
**Option C:** This option is incorrect because the Paul Bunnell test is not used to diagnose fungal infections such as histoplasmosis.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the Paul Bunnell test is not 100% specific for IMN, and a positive result should be interpreted in the context of clinical presentation and other laboratory findings.
**Correct Answer:** C. Epstein-Barr virus.