**Question:** Elek test is used for detecting toxins of the following infection:

A. Anthrax
B. Plague
C. Tetanus
D. Smallpox

**Correct Answer:** **C. Tetanus**

**Core Concept:** The Elek test is a serological test used to detect the presence of tetanus toxin antibodies in the blood, which is a sign of a previous tetanus infection or vaccination.

**Why the Correct Answer is Right:** The tetanus infection is caused by the bacterium Clostridium tetani, which produces a potent neurotoxin called tetanospasmin. The immune system responds by producing specific antibodies against this toxin. The Elek test detects these antibodies in the blood, indicating that the person has either had a previous tetanus infection or has been vaccinated against tetanus.

**Why Each Wrong Option is Incorrect:**

A. Anthrax: The Elek test is specific for tetanus toxin, not anthrax toxin. Anthrax is caused by Bacillus anthracis and produces anthrax toxins, not tetanus toxins.

B. Plague: Plague is caused by Yersinia pestis and produces plague toxins, not tetanus toxins. The Elek test is not used for plague diagnosis.

D. Smallpox: Smallpox is caused by variola virus and produces viral proteins, not bacterial toxins. The Elek test is not used for smallpox diagnosis.

**Clinical Pearl:** The Elek test is a valuable tool for diagnosing tetanus and assessing immunity status in a patient after tetanus vaccination or prior tetanus infection. It helps in determining the need for tetanus booster vaccines and can guide tetanus prophylaxis in patients with suspected or confirmed tetanus.