Clients taking metformin (Glucophage) should be monitored for this rare but potentially fatal adverse effect:

The correct answer is D: Lactic acidosis. Metformin can lead to the accumulation of lactic acid in the body, causing lactic acidosis. This is a rare but serious adverse effect that can be fatal if not recognized and treated promptly. Monitoring for symptoms such as abdominal pain, weakness, difficulty breathing, and confusion is crucial. Respiratory acidosis (A), hyperlipidemia (B), and hypertension (C) are not directly associated with metformin use. Lactic acidosis is the most significant adverse effect to monitor for in clients taking metformin.