**Question:** Investigation of choice for the diagnosis of diptheria carrier is:

**Core Concept:** Diptheria is a bacterial infection caused by Corynebacterium diphtheriae, which presents as a toxin producing infection. The toxin is responsible for the characteristic symptoms and complications of the disease. In some cases, individuals can be carriers of the bacteria without showing symptoms. Diagnosing diptheria carriers is essential for understanding the transmission dynamics and implementing preventive measures.

**Why the Correct Answer is Right:**

The correct answer, **C. Antigen detection**, is the preferred method for diagnosing diptheria carriers. Antigen detection involves the identification of antigens produced by Corynebacterium diphtheriae in the carrier's samples. This method is reliable and sensitive, allowing for the detection of low levels of toxin-producing bacteria.

**Why Each Wrong Option is Incorrect:**

A. **Culture (A. Culture):** Culture is the process of isolating microorganisms from a clinical sample. While it can detect the presence of Corynebacterium diphtheriae, it might not be sensitive enough to identify carriers with low levels of toxin production.

B. **Serology (B. Serology):** Serology tests focus on detecting antibodies against the bacteria or toxin, not the bacteria themselves. These tests are useful for diagnosing active infections but may not accurately identify carriers.

D. **Nasal swab (D. Nasal swab):** While a nasal swab is a common sample used for diptheria diagnostic testing, the detection of antigens is more reliable and sensitive than simply assessing the presence of bacteria in the sample. Antigen detection allows for a more accurate identification of carriers.

**Clinical Pearl:**

Diagnosing diptheria carriers is crucial for understanding transmission dynamics and preventing outbreaks. Antigen detection is the gold standard for this purpose, as it can identify individuals with low levels of toxin production, which may not be detected by culture or serology methods.

Remember, to effectively prevent and control diptheria outbreaks, it is essential to identify and isolate carriers, and antigen detection is the most reliable method for this purpose.