**Core Concept:** The process by which the body reduces the amount of active drug in the bloodstream after absorption is known as pharmacokinetics. This includes four main phases: absorption, distribution, biotransformation, and elimination. Biotransformation is the process of changing the drug molecule into another form through enzymatic reactions, primarily in the liver. This is followed by elimination, which is the phase of excreting the drug's waste products or the unchanged drug from the body.

**Why the Correct Answer is Right:** The correct answer, **D. Biotransformation**, refers to the process of altering the drug molecule into other forms through enzymatic reactions, primarily in the liver. This is a crucial step in pharmacokinetics, as it allows the body to neutralize or eliminate the drug, preventing potential toxicity and maintaining therapeutic levels.

**Why Each Wrong Option is Incorrect:**

A. **Absorption:** This is the process through which drugs enter the bloodstream after being ingested or administered. It is not the process responsible for reducing the drug concentration in the body.

B. **Distribution:** This refers to the movement of drugs from the bloodstream to other body tissues and fluids, but it does not reduce drug concentrations in the body.

C. **Elimination:** While elimination is indeed a part of pharmacokinetics, the correct answer should focus on the process that reduces drug concentrations, which is **Biotransformation**. Elimination refers to the excretion of drugs from the body, not the reduction of drug concentrations.

**Clinical Pearl:** Understanding pharmacokinetics is essential for safe and effective drug therapy. Ensure to consider absorption, distribution, biotransformation, and elimination phases when prescribing medications and monitoring patients for drug response and potential adverse effects.