**Core Concept**
The concept of varying drug responsiveness in different individuals is known as pharmacokinetic variability, which arises from differences in drug absorption, distribution, metabolism, and excretion (ADME). This variability can lead to differences in the concentration of the drug at its site of action, resulting in different responses to the same dose.

**Why the Correct Answer is Right**
The correct answer is related to the concept of pharmacokinetic variability, specifically the difference in drug metabolism. **Pharmacogenetics** is the study of how genetic variations affect an individual's response to drugs. It involves the analysis of genetic differences in drug-metabolizing enzymes, such as cytochrome P450, which can affect the rate of drug metabolism and, consequently, the concentration of the drug at its site of action. This can lead to variations in drug responsiveness in different individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pharmacodynamics, the study of the biochemical and physiological effects of drugs, does not directly explain variations in drug responsiveness due to differences in drug metabolism.

**Option B:** This option is incorrect because pharmacological variation, which refers to the differences in drug response due to variations in drug-receptor interactions, does not directly explain variations in drug responsiveness due to differences in drug metabolism.

**Option C:** This option is incorrect because pharmacological variability, which refers to the differences in drug response due to variations in drug-receptor interactions, does not directly explain variations in drug responsiveness due to differences in drug metabolism.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pharmacogenetics can affect an individual's response to a wide range of drugs, including those used for cardiovascular disease, cancer, and psychiatric disorders.

**Correct Answer: D. Pharmacogenetics**