Which instruction is most important for the nurse to provide a client who receives a new prescription for risedronate sodium to treat osteoporosis?

A. Annual bone density tests are useful but not the immediate priority for medication safety. B. Weight-bearing exercise supports bone health but is secondary to medication administration instructions. C. Remaining upright for at least 30 minutes after taking risedronate sodium is crucial to prevent esophageal irritation or ulceration, a known side effect of bisphosphonates. D. Increasing calcium intake is important for osteoporosis but less critical than preventing medication-related adverse effects initially.