Final answer:
The gatekeeper in a Health Maintenance Organization (HMO) is the primary care physician, whose role is crucial in coordinating care and managing resources to prevent overutilization and control costs.
Step-by-step explanation:
The gatekeeper in a Health Maintenance Organization (HMO) is the primary care physician. In an HMO, medical care providers are reimbursed based on the number of patients managed, not solely by the services provided. The primary care physician's role is to coordinate patient care and make decisions about which services are necessary, effectively managing resources and controlling costs. This structure is designed to minimize moral hazard by aligning the incentives of healthcare providers with the goals of cost control and efficient use of medical services, as opposed to a fee-for-service system where providers are paid more for additional services rendered.