## **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 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 immunological response to the toxin.

## **Why the Correct Answer is Right**
A positive Schick test indicates that the individual lacks sufficient immunity to diphtheria toxin, meaning they are not immune or have low levels of antibodies against the toxin. This is because the immune system does not produce enough antitoxin to neutralize the injected toxin, leading to a localized reaction at the injection site, typically redness, swelling, and induration. The correct answer, therefore, relates to the lack of immunity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because a negative Schick test actually indicates immunity to diphtheria, not susceptibility.
- **Option B:** This option might seem plausible but does not directly relate to the interpretation of the Schick test results. The Schick test specifically assesses immunity to diphtheria.
- **Option C:** Similar to Option B, this does not accurately describe the outcome of a positive Schick test.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a positive Schick test signifies a lack of immunity, indicating the need for vaccination or reinforcement of immunization against diphtheria. This is particularly important in clinical settings to prevent outbreaks of diphtheria.

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