Leptin Is a Circulating Hormone That Is Produced by Fat

Leptin is a circulating hormone that is produced by fat cells and plays a role in body metabolism and obesity. It normally binds to receptors in the brain and inhibits appetite. Studies have demonstrated that when obese mice that are leptin deficient are injected with leptin they quickly lose their excess weight. Interestingly, however, many overweight people have high levels of leptin in their bloodstream. Why do you think that the high levels of leptin in obese individuals are insufficient to curb their appetite?

A) Obese people who have high levels of leptin may have a leptin receptor with decreased sensitivity.
B) In obese people with high levels of leptin, the leptin is a non-functional second messenger.
C) Paracrine signaling may be disrupted in obese people with high levels of leptin.
D) The form of leptin released by obese people may lack protein kinase activity.