**Question:** Patch test is read after –

A. 24 hours
B. 48 hours
C. 72 hours
D. 96 hours

**Correct Answer:**

**Core Concept:** Patch testing is a diagnostic procedure used in dermatology to identify skin allergies by applying various substances to the skin and observing the patient's reaction over time.

**Why the Correct Answer is Right:** Patch tests are read after exposing the skin to allergen substances for a specific period to determine if a patient has developed a delayed type IV hypersensitivity reaction. In this case, the correct answer is **B. 48 hours** because it is the time period after patch application when most reactions occur. This is due to the development of a delayed type IV hypersensitivity reaction, which typically presents after 24-48 hours.

**Why Each Wrong Option is Incorrect:**

A. **24 hours** is too early for a delayed type IV hypersensitivity reaction to develop.
C. **72 hours** is too long for the reactions to be read, as reactions may fade or become less distinct over time.
D. **96 hours** is longer than necessary, as reactions usually peak between 24-48 hours.

**Why the Correct Answer is Right:**

**Clinical Pearl:** Patch testing allows dermatologists to identify specific allergens causing skin reactions, which is crucial for proper diagnosis and treatment of allergic skin conditions.

In summary, the correct answer is **B. 48 hours** as it represents the optimal time for reading patch test results, allowing for the development of delayed type IV hypersensitivity reactions due to allergen exposure.