**Core Concept**
The epithalamus is a region of the diencephalon in the brain, playing a crucial role in the regulation of various physiological processes, including reproductive and sleep-wake cycles. It is composed of several structures, including the pineal gland, the posterior commissure, and the habenular nuclei.

**Why the Correct Answer is Right**
The pineal gland is a key component of the epithalamus, responsible for producing melatonin, a hormone regulating the sleep-wake cycle. The posterior commissure is a bundle of nerve fibers connecting the two hemispheres of the brain, facilitating the transmission of signals. The habenular nuclei are involved in the modulation of emotional responses and the regulation of the hypothalamic-pituitary-adrenal axis. These structures work together to maintain various physiological processes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the epithalamus indeed includes the pineal gland.
**Option B:** This option is incorrect as the posterior commissure is a part of the epithalamus.
**Option C:** This option is incorrect as the habenular nuclei are also a component of the epithalamus.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the pineal gland's production of melatonin is regulated by the suprachiasmatic nucleus (SCN), which responds to light exposure from the environment.

**Correct Answer: D.**