**Core Concept**
The question is about the initial step in formulating a hypothesis when dealing with an unknown etiology, which is a fundamental concept in epidemiology and medical research. This involves identifying and analyzing potential risk factors, which is crucial in understanding the disease's underlying mechanisms.

**Why the Correct Answer is Right**
The first step in formulating a hypothesis when dealing with an unknown etiology is to conduct a thorough literature review and gather existing knowledge about the disease. This involves identifying all known risk factors, including genetic, environmental, and lifestyle factors. This step is essential in narrowing down potential causes and forming a hypothesis that can be tested through further research. By analyzing existing data and research, investigators can identify potential patterns and correlations that may lead to a hypothesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not represent a systematic approach to identifying potential causes of a disease. While interviewing patients can provide valuable information, it is not the first step in formulating a hypothesis.

**Option B:** This option is incorrect because it is more of an experimental approach, which may not be feasible or practical in the initial stages of hypothesis formation.

**Option C:** This option is incorrect because it is a more advanced step in hypothesis testing, which involves designing and implementing experiments to test the hypothesis.

**Clinical Pearl / High-Yield Fact**
A thorough literature review is essential in formulating a hypothesis and should include a critical analysis of existing research, identification of knowledge gaps, and a clear understanding of the disease's epidemiology.

**Correct Answer: A. Conducting a thorough literature review and gathering existing knowledge about the disease.**