What laboratory finding is most indicative of renal tubular acidosis in a patient displaying metabolic acidosis?

The correct laboratory finding indicative of renal tubular acidosis (RTA) in a patient with metabolic acidosis is low bicarbonate levels. In RTA, kidneys fail to excrete hydrogen ions properly or do not reabsorb bicarbonate effectively, leading to a primary metabolic acidosis characterized by low bicarbonate levels in the serum. This drop in bicarbonate is essential to diagnosing metabolic acidosis caused specifically by RTA rather than other causes.

In renal tubular acidosis, despite the presence of a metabolic acidosis, the bicarbonate levels can remain low because the renal tubules are either not reabsorbing bicarbonate as they should or are unable to secrete hydrogen ions, which would allow bicarbonate to be preserved in the serum.

Additional findings in RTA can include a variety of urine pH results based on the specific type of RTA, but the crucial indicator of the acidosis from a metabolic perspective is the decrease in bicarbonate. Other options, while they can be involved in metabolic acidosis or related renal issues, do not specifically point to the underlying mechanism of RTA as directly as the measurement of bicarbonate levels.