The nurse teaches a client to store unopened insulin vials. Which statement shows correct understanding?

Rationale: A. Insulin is a protein-based hormone. Freezing causes crystal formation and denaturation of protein structure, which destroys the insulin's ability to lower blood glucose. Using frozen insulin can lead to hyperglycemia due to ineffective doses. Therefore, freezing is contraindicated. B. Freezing insulin, whether opened or unopened, renders it inactive. Proper storage is essential to maintain potency; storing in a freezer is a common error that can result in ineffective therapy and poor glycemic control. C. Refrigeration (2–8°C / 36–46°F) preserves insulin stability and potency until the expiration date. Most insulin formulations remain effective for the full shelf life when stored this way. Keeping unopened insulin in the fridge also reduces the risk of accidental temperature fluctuations that can degrade the medication. D. Once insulin is opened, most vials or pens can be kept at room temperature (up to 30°C / 86°F) for a limited period (usually 28 days for many types, longer for some formulations) without loss of efficacy. Room temperature storage: Reduces injection discomfort (cold insulin can cause burning), maintains potency if within manufacturer-specified duration and must still avoid extremes of heat or freezing