**Core Concept**
The first step in investigating an epidemic involves identifying the cause of the outbreak, which is crucial for implementing effective control measures. This process requires a systematic approach to gather information, identify risk factors, and determine the mode of transmission. The initial step is to define the problem and gather data on the affected population.

**Why the Correct Answer is Right**
The first step in investigating an epidemic is to define the problem, which involves gathering information on the affected population, including demographic characteristics, symptoms, and exposure history. This information is essential for identifying potential risk factors and determining the mode of transmission. The epidemiologist must also determine the magnitude of the outbreak, including the number of cases and the geographic distribution of affected individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the initial step in investigating an epidemic. While surveillance is an essential component of epidemic investigation, it is not the first step.

**Option B:** This option is incorrect because it is a specific method used in the investigation of an epidemic, but it is not the first step. Laboratory diagnosis is a critical component of epidemic investigation, but it is typically performed after the initial data collection and case definition.

**Option C:** This option is incorrect because it is a control measure used to prevent the spread of disease, but it is not the first step in investigating an epidemic.

**Clinical Pearl / High-Yield Fact**
The first step in investigating an epidemic is often referred to as the "epidemiologic triad," which consists of host, agent, and environment. Understanding the relationship between these three components is essential for identifying the cause of the outbreak and implementing effective control measures.

**Correct Answer:** A. Define the problem and gather data on the affected population.