What entity specializes in sharing medical data among life insurance companies?

The Medical Information Bureau (MIB) is the correct choice because it serves as a critical resource for life insurance companies looking to share important medical information about prospective policyholders. The MIB functions as a membership-based organization that provides insurers with a way to access medical records and other relevant data that can help them assess the risk associated with issuing a life insurance policy. By pooling information on medical conditions and underwriting risks, the MIB helps insurance companies make more informed decisions regarding applications.

This system helps in preventing fraud and ensures that insurers have a more comprehensive view of an applicant's health history, allowing them to provide more accurate underwriting and pricing of life insurance policies. The confidentiality and privacy of this medical data are also paramount, and the MIB follows strict regulations to protect consumers.

Other entities mentioned in the options, such as the National Insurance Association, State Health Board, and Insurance Medical Service, do not specifically focus on the sharing of medical data among life insurance companies in the same way that the MIB does. Thus, the MIB's specialized role in this area clearly distinguishes it as the appropriate answer.