Leptin is a hormone that is produced by fat cells and plays a role in body metabolism and obesity. Studies have demonstrated that when obese mice are injected with leptin they quickly lose their excess weight. Interestingly, however, many overweight people have high levels of leptin in their bloodstream. Which of the following explanations is most consistent with this observation?
a. Paracrine signaling may be disrupted in obese people with high levels of leptin. b. Obese people who have high levels of leptin may have a defective leptin receptor. c. The form of leptin released by obese people may lack protein kinase activity. d. In obese people with high levels of leptin, the leptin is binding to the wrong receptor.