Micro dosing studies done in which phase of clinical trials?

**Core Concept**
The question pertains to the phase of clinical trials where microdosing studies are conducted, which is an essential aspect of **drug development**. Microdosing involves the administration of doses so low that they are unlikely to produce whole-body effects, but high enough to allow the cellular response to be studied. This concept is crucial in **pharmacokinetics** and **pharmacodynamics**.

**Why the Correct Answer is Right**
Microdosing studies are typically done in **Phase I** of clinical trials. This phase is primarily concerned with assessing the drug's safety, tolerability, **pharmacokinetics**, and **pharmacodynamics** in a small group of healthy volunteers. Microdosing in Phase I helps to quickly and cost-effectively evaluate the absorption, distribution, metabolism, and excretion (ADME) of new chemical entities.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not the correct phase for microdosing studies, as Phase I is where these studies are typically conducted.
**Option C and D:** These phases focus on efficacy and large-scale safety, respectively, and are not where microdosing studies are primarily conducted.

**Clinical Pearl / High-Yield Fact**
Remember, microdosing studies are a part of **exploratory IND** studies, which are a type of Phase I trial. They are particularly useful for understanding how a drug behaves in the human body before proceeding to larger, more expensive trials.

**Correct Answer:** Correct Answer: B. Phase I.