Which two comorbidities contraindicate the use of Metformin?

The combination of congestive heart failure (CHF) and renal disease contraindicates the use of Metformin due to the increased risk of lactic acidosis, a potential serious side effect of this medication. Metformin is primarily eliminated by the kidneys, and when renal function is compromised, the drug can accumulate in the body, raising the risk of lactic acidosis. Additionally, CHF can impact renal perfusion and consequently worsen renal function, further increasing these risks. Therefore, in patients with both CHF and renal disease, the use of Metformin is advised against to ensure patient safety.

The other possible combinations of comorbidities presented in the question do not have the same level of direct clinical implications concerning Metformin's safety profile. For instance, while hypertension and renal disease are both significant health concerns, the specific risk associated with Metformin primarily relates to severe renal impairment. Similarly, while chronic lung disease may complicate a patient’s overall health status, it does not directly contraindicate Metformin use in the same way that CHF and renal disease do.