**Core Concept**
Prader-Willi syndrome (PWS) is a genetic disorder characterized by a deletion or loss of function of genes on chromosome 15 inherited from the father. PWS is associated with various endocrine abnormalities, including hormonal imbalances that affect growth and metabolism.

**Why the Correct Answer is Right**
The correct answer is D. Ghrelin. Ghrelin is a hormone produced by the stomach that stimulates appetite and promotes food intake. In Prader-Willi syndrome, levels of ghrelin are increased, which contributes to the excessive hunger and eating behavior characteristic of the condition. This increased ghrelin also leads to increased body weight and obesity in individuals with PWS.

**Why Each Wrong Option is Incorrect**
**Option A:** LH (Luteinizing hormone) levels are typically decreased in Prader-Willi syndrome, not increased. LH plays a crucial role in the regulation of the reproductive axis, and its deficiency contributes to the hypogonadism and infertility often seen in individuals with PWS.
**Option B:** FSH (Follicle-stimulating hormone) levels are also decreased in PWS, similar to LH. FSH is essential for the development and maturation of ovarian follicles in females and spermatogenesis in males.
**Option C:** TSH (Thyroid-stimulating hormone) levels may be normal or slightly decreased in PWS, but they are not typically increased. TSH regulates thyroid hormone production, which is often impaired in individuals with PWS, leading to hypothyroidism.

**Clinical Pearl / High-Yield Fact**
Prader-Willi syndrome is characterized by a unique constellation of physical and behavioral features, including short stature, hypotonia, and obesity. Individuals with PWS often have a characteristic "happy" or "contented" facial expression, which is a useful clinical clue for diagnosis.

**✓ Correct Answer: D. D. Ghrelin**