**Core Concept**
The Monospot test is a rapid diagnostic test used to detect the presence of heterophile antibodies in a patient's serum, which are associated with infectious mononucleosis (IMN) caused by Epstein-Barr virus (EBV).

**Why the Correct Answer is Right**
The Monospot test detects the heterophile antibodies that are produced in response to the EBV infection. These antibodies are directed against the antigens of sheep erythrocytes, which are used as a substrate in the test. When the heterophile antibodies bind to the sheep erythrocytes, they cause agglutination, indicating a positive result. The test is commonly used to diagnose infectious mononucleosis in patients who present with symptoms such as fever, sore throat, and lymphadenopathy.

**Why Each Wrong Option is Incorrect**
**Option A:** The Monospot test is not used to diagnose streptococcal infections, which are typically diagnosed using rapid streptococcal antigen tests or throat cultures.
**Option B:** The Monospot test is not used to diagnose Lyme disease, which is diagnosed using serological tests such as the ELISA or Western blot.
**Option C:** The Monospot test is not used to diagnose rubella, which is diagnosed using serological tests such as the ELISA or rapid antigen tests.

**Clinical Pearl / High-Yield Fact**
The Monospot test has a high sensitivity and specificity for diagnosing infectious mononucleosis, but it can be negative in some cases, especially in the early stages of the disease. Therefore, a negative result does not rule out the diagnosis, and clinical correlation with symptoms and other diagnostic tests is essential.

**Correct Answer:** C. Infection caused by Epstein-Barr virus.