## **Core Concept**
The specificity of a screening test is a measure of its ability to correctly identify those without the disease, i.e., non-diseased persons. It is defined as the proportion of true negatives (non-diseased persons correctly identified as negative) among all non-diseased persons.

## **Why the Correct Answer is Right**
A specificity of 90% for a screening test means that out of 100 non-diseased persons, 90 will test negative (true negatives) and 10 will test positive (false positives). Therefore, 90% specificity implies that 90% of non-diseased persons will give a **negative** test result.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incomplete and cannot be evaluated.
- **Option B:** This option suggests a positive test result, which is incorrect because a 90% specificity means 90% of non-diseased persons will test negative, not positive.
- **Option C:** This is the correct statement related to specificity; hence, it's not an incorrect option but the basis of our explanation.
- **Option D:** This option is incomplete and cannot be evaluated.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **specificity** helps in ruling in a disease when a test is positive. A highly specific test (close to 100% specificity) is useful for confirming a diagnosis because a positive result accurately indicates the presence of the disease. Conversely, a **sensitive** test is better for ruling out a disease when it's negative.

## **Correct Answer:** B. negative test result.