**Core Concept**
The question is testing the concept of drug potency and its relationship with the minimum effective concentration required to produce a desired therapeutic effect. A drug's potency is measured by its ability to produce a therapeutic effect at a low concentration.

**Why the Correct Answer is Right**
A drug that can be used in low concentrations is said to have high potency. This means that the drug has a high affinity for its receptor and can elicit a response at a lower concentration. For example, a drug that can effectively treat hypertension at a dose of 0.1 mg/kg is more potent than a drug that requires a dose of 10 mg/kg to achieve the same effect. The high potency of a drug is often due to its ability to bind to a specific receptor with high affinity, such as a high-affinity agonist or a drug that can selectively activate a specific signaling pathway.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the question and does not provide any information about the properties of a drug that would enable it to be used in low concentrations.
* **Option B:** This option is incorrect because a drug's efficacy refers to its ability to produce a maximum effect, not its ability to be used in low concentrations. A drug can have high efficacy but still require high concentrations to achieve its effect.
* **Option C:** This option is incorrect because a drug's duration of action refers to the length of time it takes for the drug to be eliminated from the body, not its ability to be used in low concentrations.

**Clinical Pearl / High-Yield Fact**
A drug's potency is often measured in terms of its ED50 (effective dose 50), which is the dose required to produce a 50% effect. A drug with a low ED50 is considered to be more potent than a drug with a high ED50.

**Correct Answer:** C. High potency.