What is a black box warning associated with Metformin?

The correct answer is associated with lactic acidosis, which is a serious condition that can occur in patients taking Metformin. This black box warning is crucial because lactic acidosis can be life-threatening and is particularly a risk in individuals with impaired renal function or in situations where the kidney is unable to effectively excrete Metformin. The mechanism behind this warning stems from the fact that Metformin decreases hepatic glucose production, which can lead to an increased risk of lactic acid build-up in certain patients, especially those who have pre-existing conditions that can exacerbate the risk.

The emphasis on lactic acidosis in the black box warning reflects the importance of monitoring kidney function before initiating therapy with Metformin and regularly thereafter. It is necessary to educate patients on recognizing early signs of lactic acidosis, which can include symptoms like unusual fatigue, muscle pain, trouble breathing, stomach discomfort, dizziness, or feeling cold.

Other options provided in the question, while they may be relevant to Metformin's side effects or contraindications, do not carry the same level of urgency or risk that lactic acidosis does, thus they do not warrant a black box warning.