Hypertension is not a complication associated with the use of metformin in renal impairment, whereas lactic acidosis is a known risk.

The incorrect answer "Acute on chronic renal failure" is not the most likely complication of metformin use in renal impairment. Lactic acidosis is the correct complication because metformin can accumulate in the body in renal impairment.

Nephrotic syndrome is a kidney disorder, but lactic acidosis is the more specific and severe risk associated with metformin use in kidney impairment.

Metformin can cause lactic acidosis in renal impairment or liver disease because it inhibits hepatic gluconeogenesis which uses lactate.