## **Core Concept**
The patch test is a diagnostic tool used to assess allergic contact dermatitis. It involves applying small amounts of potential allergens to the skin and observing the reaction after a certain period. The timing of reading the patch test is crucial for accurate interpretation.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the patch test is read after 48-72 hours. This timeframe allows for the development of a delayed hypersensitivity reaction, which is characteristic of allergic contact dermatitis. The reaction involves T-cell activation and the subsequent inflammatory response, which typically takes 48-72 hours to peak.

## **Why Each Wrong Option is Incorrect**
* **Option A:** is too early for reading a patch test because it doesn't allow sufficient time for the development of a delayed hypersensitivity reaction.
* **Option B:** is also incorrect because, although some reactions might be visible, the peak reaction time is usually later.
* **Option D:** is too late; reactions typically start to resolve by this time, and reading too late can lead to false-negative results.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that patch test readings should be done at 48-72 hours, and sometimes a second reading is recommended at 96-120 hours to capture late reactions. This ensures a more accurate diagnosis of allergic contact dermatitis.

## **Correct Answer:** . 72 hours.