As retailers struggle to keep up with changing shopping trends, mall operators across the U.S. are looking to fill spaces left empty by the likes of Sears and RadioShack. One promising new group of tenants: walk-in medical clinics, staffed by doctors who can treat common ailments such as pink eye and minor injuries like sprains and burns. The clinics—regional chains such as City Practice Group of New York and national ones like Concentra, the largest urgent-care organization in the U.S.—are a growing segment of the medical retail industry, says Scott Mason, executive managing director of Cushman & Wakefield's health-care group.