Which of the following is a covered service under Part B?

Part B of Medicare specifically covers medically necessary services and supplies for the diagnosis and treatment of health conditions. Among the options provided, blood transfusions are indeed a covered service under Part B because they are typically considered necessary for treating medical conditions such as anemia or severe blood loss. Medicare Part B includes various outpatient services, and blood transfusions are classified under the essential medical services that can be administered on an outpatient basis.

In contrast, routine physical exams, which are preventive in nature, are not typically covered under Part B unless they are part of a specific wellness visit that meets certain criteria. Inpatient care, which involves staying overnight in a hospital, falls under Medicare Part A, not Part B. Cosmetic surgery, unless medically necessary for reconstructive purposes following an injury or disease, is generally not covered by Medicare at all. Thus, blood transfusions stand out as the only clearly covered service under Part B based on the definitions of Medicare coverage.