## **Core Concept**
The Schick test is a method used to determine whether an individual is susceptible to diphtheria, a bacterial infection caused by *Corynebacterium diphtheriae*. It assesses the presence of immunity to diphtheria toxin. The test involves injecting a small, harmless amount of diphtheria toxin into the skin.

## **Why the Correct Answer is Right**
A positive Schick test indicates that the individual does not have sufficient immunity to diphtheria toxin and is therefore susceptible to diphtheria infection. This is because the body's immune system does not produce enough antibodies against the toxin to neutralize it, leading to a localized reaction at the injection site. The reaction, which typically occurs within 48-72 hours, signifies a lack of protective immunity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because a negative Schick test actually indicates immunity to diphtheria, meaning the individual has sufficient antibodies to protect against the toxin.
- **Option B:** This option is incorrect as it does not accurately describe the implication of a positive Schick test.
- **Option C:** This option is incorrect because it is not relevant to the interpretation of a positive Schick test.

## **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 booster shots to enhance immunity. The Schick test is less commonly used today due to widespread vaccination programs but remains a historical and educational tool in assessing immunity.

## **Correct Answer:** . Susceptibility to diphtheria.