**Core Concept**
Orexigenic agents are substances that stimulate appetite and food intake. Ghrelin, a hormone produced primarily in the stomach, is a key orexigenic peptide that acts on the hypothalamus to increase hunger.

**Why the Correct Answer is Right**
Ghrelin is the primary orexigenic hormone in the body. It is secreted by the stomach, especially when the stomach is empty, and binds to growth hormone secretagogue receptors (GHSR) in the hypothalamus. This activates the arcuate nucleus, stimulating neuropeptide Y (NPY) and agouti-related peptide (AGRP) neurons, which promote feeding behavior. Ghrelin levels rise before meals and fall after eating, making it a key regulator of appetite.

**Why Each Wrong Option is Incorrect**
Option A: Serotonin is **anorexigenic**, acting in the hypothalamus to suppress appetite. It reduces food intake and is involved in satiety.
Option C: OCX (Orexin) is **not orexigenic**; it is actually **anorexigenic** and promotes wakefulness, not hunger. It is involved in arousal and feeding regulation but does not stimulate appetite.
Option D: GA (Agouti-related peptide) is **anorexigenic** and acts as an antagonist of melanocortin receptors, inhibiting appetite, not stimulating it.

**Clinical Pearl / High-Yield Fact**
Ghrelin is the only known hormone that directly stimulates hunger. Its levels are highest before meals and lowest after eating. This makes it a key target in treating obesity and eating disorders.

✓ Correct Answer: B. Ghrelin