**Core Concept**
The secondary attack rate (SAR) is a measure of the spread of an infectious disease within a defined population, typically calculated as the number of new cases that occur among contacts within a specified time period following exposure to the primary case, divided by the total number of exposed individuals. The denominator in the SAR calculation is crucial, as it represents the population at risk of infection.

**Why the Correct Answer is Right**
The correct denominator for calculating the SAR includes all individuals who were in close contact with the primary case and were at risk of developing the disease. This typically includes household members, intimate contacts, and anyone else who may have been in close proximity to the primary case, such as healthcare workers or individuals who shared a meal or space with the primary case. The denominator does **not** include individuals who were not exposed to the primary case, such as those who were not in close contact or were isolated at the time of exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as this would include individuals who were not exposed to the primary case and were not at risk of developing the disease.
**Option B:** Incorrect, as this would exclude individuals who were exposed but did not develop symptoms, which would artificially inflate the SAR.
**Option C:** Incorrect, as this would include individuals who were not in close contact with the primary case and were not at risk of developing the disease.

**Clinical Pearl / High-Yield Fact**
When calculating the SAR, it's essential to clearly define the population at risk, including all individuals who were in close contact with the primary case and were at risk of developing the disease. This helps to accurately assess the transmission dynamics of the disease and inform public health interventions.

**Correct Answer:** C. The total number of exposed individuals.