A nurse teaches a patient to take a prescribed glucocorticoid first thing in the morning because:

Taking a prescribed glucocorticoid first thing in the morning mimics the body's natural diurnal rhythm of cortisol secretion. Cortisol levels typically peak in the early morning and gradually decline throughout the day. Administering glucocorticoids at this time aligns the medication's action with the body's own cortisol production, which can enhance the drug's effectiveness and minimize potential side effects. This approach can help to manage conditions in a way that closely resembles the body's normal physiological processes.

The timing also helps to reduce the risk of adrenal suppression, as the body can better manage the levels of cortisol when medication is administered in alignment with natural secretion patterns. Thus, option A is not only correct, but it reflects a fundamental principle of pharmacology concerning glucocorticoid therapy.