**Core Concept**
Estrogen beta receptors, also known as G protein-coupled estrogen receptor 1 (GPER1), are a subtype of estrogen receptors that mediate rapid, non-genomic effects of estrogen. These receptors are distinct from estrogen alpha receptors (ERα) and estrogen receptor beta (ERβ).

**Why the Correct Answer is Right**
Estrogen beta receptors are primarily found in the brain, heart, and blood vessels, where they play a role in regulating cardiovascular function and modulating the immune system. Activation of GPER1 by estrogen leads to the activation of various signaling pathways, including the MAPK and PI3K/AKT pathways, resulting in rapid effects on cellular function. In contrast to ERα, which is primarily nuclear and mediates genomic effects, GPER1 is membrane-bound and mediates rapid, non-genomic effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because estrogen beta receptors are not primarily found in the liver, where they would play a role in regulating lipid metabolism and insulin sensitivity.
**Option B:** This option is incorrect because while estrogen receptors are found in the uterus, the specific subtype of estrogen beta receptors (GPER1) is not primarily associated with uterine function.
**Option C:** This option is incorrect because estrogen beta receptors are not primarily found in the kidneys, where they would play a role in regulating electrolyte balance and blood pressure.

**Clinical Pearl / High-Yield Fact**
GPER1 is a key mediator of the rapid effects of estrogen on the cardiovascular system, and its activation has been implicated in the regulation of blood pressure and cardiac function.

**Correct Answer:** D.