## Core Concept
Clinical trials are divided into phases to assess the safety and efficacy of new drugs or interventions. Each phase involves a specific group of participants and aims to answer particular research questions. Healthy human volunteers are typically involved in the early phases of clinical trials.

## Why the Correct Answer is Right
Phase 1 clinical trials are primarily concerned with assessing the safety, tolerability, pharmacokinetics, and pharmacodynamics of a drug. These trials are usually conducted with a small group of healthy volunteers, although in some cases, patients with the condition of interest may be studied. The involvement of healthy volunteers in Phase 1 trials helps researchers understand how the drug is processed in the body and its initial safety profile.

## Why Each Wrong Option is Incorrect
**Option A:** Phase 2 trials involve patients with the condition the drug aims to treat. These trials are focused on the efficacy and side effects of the drug in patients, not healthy volunteers.

**Option B:** Phase 3 trials are large-scale trials that seek to confirm efficacy, monitor side effects, compare the drug to commonly used treatments, and collect information that will allow the experimental drug to be used safely. Patients, not healthy volunteers, are typically involved in this phase.

**Option C:** Phase 4 trials, also known as post-marketing surveillance trials, are conducted after a drug has received permission to be sold by the pertinent regulatory authorities. These trials are often required by regulatory authorities or are undertaken by the sponsoring company for competitive or other reasons. They may involve patients or sometimes healthy volunteers but are not the primary phase for healthy volunteers.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Phase 1 clinical trials** are where the initial assessment of safety and pharmacokinetics is made, often involving **healthy volunteers**. This phase is critical for understanding the drug's safety profile and how it is metabolized in the human body.

## Correct Answer: B. Phase 1.