What should the nurse instruct a client prescribed metformin to report?

The correct choice is to instruct the client prescribed metformin to report severe abdominal pain. This is crucial because metformin can cause gastrointestinal side effects, and severe abdominal pain may indicate a more serious condition, such as lactic acidosis, which is a rare but potentially life-threatening side effect of the medication. Lactic acidosis can occur when there is an accumulation of lactic acid in the body, often related to underlying conditions or improper use of metformin.

While increased thirst, decreased appetite, and frequent urination may have relevance in the context of diabetes management, they are not as urgent or indicative of a serious complication as severe abdominal pain. Increased thirst could suggest hyperglycemia, decreased appetite might occur due to various factors including side effects, and frequent urination could be a result of the diabetes itself. Therefore, the emphasis on reporting severe abdominal pain is vital for client safety and early detection of potential complications associated with metformin use.