Receptor reserve, also known as spare receptors, means

• A. Maximal effect requires occupation of all receptors by agonist
• B. Receptors are not involved in achieving maximal effect
• C. Maximal effect can be achieved without occupying all receptors
• D. Only high concentrations of agonist yield a maximal response

Answer: (C)

Receptor reserve means there are more receptors in a cell or tissue than are required to produce the maximal response. Because of this surplus, the system can achieve the full effect without needing to occupy every receptor.

This happens because receptor activation is amplified downstream: activating a subset of receptors can trigger a cascade of signals strong enough to reach Emax. As a result, only a fraction of receptors need to be occupied to generate the maximal response, so the system can show full effect even though not all receptors are bound by the agonist.

That’s why the statement describing maximal effect achievable without occupying all receptors is the best fit. It reflects that the presence of spare receptors allows full efficacy at less-than-full receptor occupancy. The other ideas—needing all receptors to be occupied, receptors not involved, or maximal response only at very high concentrations—don’t align with how receptor reserve enables a full response at partial occupancy.