**Question:** Leptin is secreted by

A. Adipocytes
B. Muscle cells
C. Liver cells
D. Neurons

**Core Concept:** Leptin is a hormone that plays a crucial role in regulating appetite, energy expenditure, and body weight. It is primarily secreted by adipocytes (fat cells) in response to increased fat mass, and its primary function is to signal satiety to the brain.

**Why the Correct Answer is Right:** Leptin is primarily secreted by adipocytes, as they are the primary site of fat storage and loss. When adipocytes expand due to increased fat mass, they secrete leptin into the bloodstream, which travels to the hypothalamus. In the hypothalamus, leptin acts on specific neurons, suppressing appetite and increasing energy expenditure to maintain body weight homeostasis.

**Why Each Wrong Option is Incorrect:**

A. Leptin is not secreted by muscle cells, as they are primarily involved in muscle contraction and not fat storage or hormone secretion.
B. Muscle cells primarily produce myokines, which are different from leptin and have distinct functions, such as regulating inflammation, immune response, and energy expenditure.
C. Leptin is not secreted by liver cells, as liver cells primarily secrete other hormones like insulin, glucagon, and bilirubin.
D. Although leptin is produced in the brain, it is not produced by neurons. Leptin is primarily produced and secreted by adipocytes, as explained above.

**Clinical Pearl:** Understanding the role of leptin is crucial for understanding obesity, anorexia, and related disorders. In obesity, adipocytes secrete excessive leptin, which may not be effectively transmitted to the brain, leading to reduced satiety sensation and increased appetite. In anorexia, the brain's sensitivity to leptin is decreased, resulting in a misinterpretation of satiety signals and increased hunger.

**Correct Answer:** Leptin is secreted by adipocytes (option A).