## **Core Concept**
The problem involves calculating the Secondary Attack Rate (SAR) of measles in a family setting. **Secondary Attack Rate** is defined as the number of new cases of a disease that occur among the contacts of a primary case within the incubation period, divided by the number of exposed contacts. Measles is a highly contagious viral disease with an incubation period of about 10-14 days.

## **Why the Correct Answer is Right**
To calculate the SAR, we first identify the primary case (the first child to develop measles) and then count the number of secondary cases that develop within the incubation period among the exposed contacts. Here, one child developed measles initially (primary case), and 6 days later, 2 other children developed measles (secondary cases). The total number of exposed contacts (susceptible individuals) is 4 (since one of the children is already immunized and thus not susceptible).

The formula for SAR is: (Number of secondary cases / Number of exposed contacts) * 100.

Given: 2 secondary cases, and 4 exposed contacts (all unimmunized siblings of the primary case).

SAR = (2 / 4) * 100 = 50%.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without calculating, we cannot assume it's incorrect, but let's evaluate other options.
- **Option B:** This would imply a different number of secondary cases or exposed contacts, which doesn't fit our calculation.
- **Option C:** Similarly, this would imply a different calculation or data not provided.
- **Option D:** This option suggests a 33.3% SAR, which would be incorrect given our calculation of 50%.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **Secondary Attack Rate** is a measure used in outbreak investigations to assess the spread of disease among contacts. For highly contagious diseases like measles, a high SAR indicates significant person-to-person transmission. Vaccination status significantly impacts SAR; in this case, the immunized child did not contribute to the spread as a susceptible contact.

## **Correct Answer:** B. 50%.