## **Core Concept**
The Schick test is a method used to determine whether a person is susceptible to diphtheria, a bacterial infection caused by *Corynebacterium diphtheriae*. It involves injecting a small amount of diphtheria toxin into the skin to observe the body's reaction. The test is based on the principle of immunology, specifically the presence or absence of immunity to diphtheria toxin.

## **Why the Correct Answer is Right**
A positive Schick test indicates that the person does not have sufficient immunity to diphtheria toxin, meaning they are susceptible to the infection. This occurs when the individual has not been vaccinated or has not developed adequate antibodies against the toxin. The test is considered positive if there is significant redness, swelling, and induration at the injection site after 48-72 hours, indicating a lack of immunity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because a negative Schick test actually indicates that the person has sufficient immunity to diphtheria, either through vaccination or previous exposure.
- **Option B:** This option is incorrect because it does not accurately describe the implication of a positive Schick test.
- **Option C:** This option might seem plausible but is incorrect because a positive Schick test specifically indicates susceptibility to diphtheria, not immunity to other infections.
- **Option D:** Without the specific details of option D, it's understood that any option not indicating susceptibility to diphtheria as a result of a positive Schick test would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a positive Schick test suggests the individual is at risk of contracting diphtheria and may benefit from vaccination or immunization against the disease. The Schick test was historically used to assess immunity in populations and guide public health vaccination efforts.

## **Correct Answer:** C. Not immune to diphtheria.