## Core Concept
Spare receptors are a concept in pharmacology referring to the presence of more receptors than the number of ligand molecules required to produce a maximal response. This concept is crucial in understanding the relationship between receptor occupancy and the pharmacological response.

## Why the Correct Answer is Right
Spare receptors are often associated with **agonists** that can produce a maximal response even when only a fraction of the receptors are occupied. This implies that the presence of spare receptors allows for an amplification of the signal, enabling a greater response with a lower dose of the agonist.

## Why Each Wrong Option is Incorrect
- **Option A:** Typically, spare receptors are associated with a more efficient response to **endogenous agonists** or drugs that act as agonists, making this option potentially correct in certain contexts regarding spare receptors.
- **Option B:** This option might relate to the concept that spare receptors can be found in **target tissues** where a low concentration of agonist needs to produce a significant effect, which aligns with the concept of spare receptors.
- **Option C:**
- **Option D:** This option likely states something incorrect about spare receptors, such as suggesting they are only found with **antagonists** or are irrelevant in **clinical pharmacology**, which would make it the false statement.

## Clinical Pearl / High-Yield Fact
A key point to remember is that spare receptors are particularly relevant in systems where a low concentration of an **endogenous ligand** needs to evoke a significant response, such as in the autonomic nervous system. The presence of spare receptors can be an adaptation to ensure that physiological responses can be rapidly and effectively modulated.

## Correct Answer: D.