Which medication could cause serious interaction with metformin?

The correct choice is Digoxin, as this medication can pose a serious interaction with metformin. When metformin is used, the risk of lactic acidosis increases, especially in patients with renal impairment. Digoxin, which is used to treat various heart conditions, can affect renal function and electrolyte balance, potentially exacerbating the side effects of metformin. This interaction requires careful monitoring of kidney function and electrolyte levels in patients taking both medications to avoid serious complications.

The other medications listed, while they may have their own interactions in different contexts, do not pose the same level of risk for serious adverse effects when used concurrently with metformin. Aspirin and ibuprofen, both non-steroidal anti-inflammatory drugs (NSAIDs), are commonly used and generally considered safe with metformin, although chronic use can increase the risk of renal impairment over time. Metoprolol, a beta-blocker, can be used in conjunction with metformin without significant interaction concerns.