## **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*. The test involves injecting a small amount of diphtheria toxin into the skin to observe the body's reaction. The result helps in assessing the individual's immunity to diphtheria.

## **Why the Correct Answer is Right**
A positive Schick test indicates that the person does not have sufficient immunity against diphtheria toxin, making them susceptible to the infection. This lack of immunity could be due to not having been vaccinated or not having had a previous infection. The test works on the principle that if a person is immune (either through vaccination or previous exposure), the injected toxin will be neutralized by circulating antibodies, and no reaction will occur. However, if there's no immunity, the toxin will cause a local reaction, indicating a positive test.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because a positive Schick test does not indicate immunity to diphtheria; rather, it shows susceptibility.
- **Option B:** This option is incorrect because a positive test does not signify that the person has been vaccinated; it actually suggests the opposite—that the person lacks sufficient immunity, possibly due to not being vaccinated or having waned immunity.
- **Option C:** This option might seem plausible but is incorrect in the context of the question. A positive Schick test actually indicates a lack of immunity, which could make a person susceptible to diphtheria, not that they are immune.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a positive Schick test indicates a person is not immune to diphtheria and is thus at risk of contracting the disease. This test was historically used to identify individuals who needed vaccination but has largely been replaced by serological tests and vaccination status assessment.

## **Correct Answer:** D. not immune to diphtheria.