Final answer:
The primary insurer for a patient covered by both a large employer group health plan and Medicare depends on employer size; for employers with 20 or more employees, the EGHP is primary, otherwise Medicare is.
Step-by-step explanation:
When a patient is covered by a large employer group health plan (EGHP) and Medicare, the primary coverage depends on the size of the employer. Generally, if the employer has 20 or more employees, the employer's health plan is considered the primary insurer, and Medicare is secondary. This is in line with the rules set out by Medicare which determine the coordination of benefits. If the group health plan is for an employer with fewer than 20 employees, Medicare usually serves as the primary payer. It's crucial for patients and healthcare providers to understand the coordination of benefits to ensure claims are submitted to the correct insurer first, which streamlines the process and avoids any delay in payments.