**Question:** All of the following hormones have receptors on the plasma membrane of target tissues except

A. Insulin
B. Thyrotropin-releasing hormone (TRH)
C. Somatostatin
D. Epinephrine

**Core Concept:**
Hormones are signaling molecules produced in endocrine glands that travel through the bloodstream to target cells or tissues where they exert their effects by binding to specific receptors. These receptors, known as hormone receptors, are typically located on the plasma membrane of target cells.

**Why the Correct Answer is Right:**
The correct answer is D. Epinephrine, also known as adrenaline, is a hormone produced by the adrenal medulla and acts on intracellular receptors, specifically the alpha and beta receptors. These receptors are not located on the plasma membrane, but rather within the cell, leading to its exclusion from the correct answer.

**Why Each Wrong Option is Incorrect:**
A. Insulin (receptor is on the plasma membrane of target cells, making it the correct choice)
B. Thyrotropin-releasing hormone (TRH) is a hypothalamic hormone that stimulates the anterior pituitary gland to produce and release thyroid-stimulating hormone (TSH) and growth hormone. TRH receptors are indeed on the plasma membrane, making it incorrect in this context.
C. Somatostatin (receptor is on the plasma membrane of target cells, making it the correct choice)

**Clinical Pearl:**
Understanding the location of hormone receptors is crucial for appreciating their actions on target cells. This knowledge is essential for understanding various physiological processes and disease states, such as in endocrinology, where hormone receptors play a fundamental role in the regulation of various physiological functions.

**Correct Answer:** D. Epinephrine