**Core Concept**
The process being referred to is the reduction of the amount of active drugs in the body after absorption into the systemic circulation. This involves the degradation, metabolism, or elimination of the drug, leading to a decrease in its efficacy or potency.

**Why the Correct Answer is Right**
The correct answer is **B. Metabolism**. This is because metabolism refers to the process by which the body breaks down drugs into their inactive metabolites, which are then excreted. This can occur in the liver via the cytochrome P450 enzyme system, or in other tissues such as the kidneys. Metabolism is an essential aspect of pharmacokinetics, as it affects the duration and intensity of a drug's action.

**Why Each Wrong Option is Incorrect**
**Option A:** Elimination is a broader term that encompasses both metabolism and excretion, but it does not specifically refer to the breakdown of drugs into inactive metabolites.
**Option C:** Excretion refers to the removal of drugs and their metabolites from the body, but it does not involve the breakdown of the drug itself.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the rate and extent of drug metabolism can be influenced by various factors, including genetic variations, age, and the presence of other medications. This can lead to significant variations in drug response and efficacy.

**Correct Answer:** B. Metabolism