Leptin: The Primary Obesity-Resistant Hormone

The question asks to identify the most prevalent hormone that resists obesity. Let's analyze the primary functions of the given options:

• Leptin: Produced by fat cells, leptin signals the brain (specifically the hypothalamus) to regulate appetite and energy expenditure. It promotes satiety (feeling full) and increases metabolism, thereby helping to prevent weight gain and resist obesity. While leptin resistance can occur in obese individuals, its physiological role is inherently anti-obesity.
• Orexin: Also known as hypocretin, orexin promotes feeding behavior and arousal. It stimulates appetite, working counter to obesity resistance.
• Insulin: Primarily regulates blood glucose levels. It also plays a role in energy storage, promoting the uptake and storage of glucose as fat. While crucial for metabolism, it is not considered the primary obesity-resistant hormone; dysregulation is linked to obesity.
• Ghrelin: Often called the "hunger hormone," ghrelin stimulates appetite and promotes fat storage. It signals the brain to increase food intake, opposing obesity resistance.

Based on their physiological roles, Leptin is the hormone whose primary function is to signal fullness and increase energy expenditure, thus acting as an obesity-resistant hormone.

Therefore, Leptin is the most prevalent obesity-resistant hormone among the choices.