**Question:** Receptor for which of the following is present intracellularly?

A. Receptor for acetylcholine
B. Receptor for histamine
C. Receptor for adrenaline
D. Receptor for dopamine

**Core Concept:**
Pharmacological receptors are specialized molecules on the cell surface that bind specific chemical messengers, known as neurotransmitters or hormones, to initiate a cellular response. There are two main categories of receptors:

1. **Extracellular:** These receptors are located on the outer surface of a cell, interacting with neurotransmitters or hormones present in the extracellular environment. Examples include receptors for acetylcholine (A), histamine (B), adrenaline (C), and dopamine (D).

**Why the Correct Answer is Right:**

Receptor for acetylcholine (A) is present intracellularly. Acetylcholine is a neurotransmitter involved in neuromuscular transmission and plays a crucial role in the parasympathetic nervous system. Intracellular receptors are located within the cell's interior, away from the extracellular environment.

**Why Each Wrong Option is Incorrect:**

1. Receptor for histamine (B) is present extracellularly, as histamine is a hormone and neurotransmitter that acts on extracellular receptors.
2. Receptor for adrenaline (C) is also present extracellularly, as adrenaline is a hormone that binds to receptors on the cell surface.
3. Receptor for dopamine (D) is present extracellularly, as dopamine is a neurotransmitter and binds to receptors on the cell surface.

**Clinical Pearl:**

Understanding the localization of receptors is crucial for understanding how drugs and hormones interact with cells. Intracellular receptors are often involved in modulating intracellular signaling pathways, while extracellular receptors mediate cell surface responses. This distinction is essential for understanding the mechanism of action of various drugs and the effects they have on cells.