**Core Concept**
Ghrelin is a peptide hormone produced by the enteroendocrine cells of the stomach, primarily in the fundus region. It plays a crucial role in regulating appetite, energy balance, and gastrointestinal function.

**Why the Correct Answer is Right**
Ghrelin stimulates appetite and increases food intake by binding to its receptor, the growth hormone secretagogue receptor type 1a (GHSR-1a). This binding leads to the activation of the phospholipase C (PLC) pathway, resulting in increased intracellular calcium levels and the subsequent release of growth hormone (GH) from the pituitary gland. Additionally, ghrelin has been shown to have prokinetic effects, enhancing gastric motility and emptying.

**Why Each Wrong Option is Incorrect**
**Option A:** Ghrelin is not primarily involved in regulating blood glucose levels. While it may have some indirect effects on glucose metabolism, its primary role is in stimulating appetite and increasing food intake.
**Option B:** Leptin is a hormone produced by adipose tissue that regulates energy balance and body weight, but it has opposite effects to ghrelin, suppressing appetite and increasing satiety.
**Option C:** Gastrin is a hormone produced by the G cells of the stomach that stimulates gastric acid secretion, but it does not have a role in regulating appetite or energy balance.

**Clinical Pearl / High-Yield Fact**
Ghrelin levels are typically highest in the fasting state and decrease after meals, which is why its administration has been explored as a potential therapeutic agent for treating obesity and cachexia.

**Correct Answer: D. Stimulating appetite and increasing food intake.**