Metformin is contraindicated in which of the following conditions?

Metformin is contraindicated in renal and liver disease primarily due to the risk of lactic acidosis. Metformin is primarily excreted by the kidneys, and any impairment in renal function can lead to an accumulation of the medication in the body, increasing the potential for adverse effects, particularly lactic acidosis, which is a rare but serious condition where lactic acid builds up in the bloodstream faster than it can be removed.

In patients with liver disease, the liver's ability to metabolize and clear various substances, including medications like metformin, is reduced, which can also contribute to the risk of lactic acidosis. Both renal and liver impairment can therefore compromise the safety profile of metformin and complicate glucose management due to increased risk.

In contrast, hypertension, chronic lung disease, and heart failure do not inherently contraindicate the use of metformin. While caution is advised in these cases, particularly heart failure, where there may be concerns regarding tissue perfusion and lactic acidosis, they do not provide the same clear contraindications as renal and liver disease. Thus, those patients may still be administered metformin with careful monitoring and consideration of their overall clinical status.