What is the most likely cause of renal tubular acidosis in an infant with failure to thrive and normal anion gap acidosis?

The most likely cause of renal tubular acidosis (RTA) in an infant presenting with failure to thrive and normal anion gap acidosis is indeed renal tubular acidosis itself. In this condition, the kidneys are unable to adequately excrete hydrogen ions or reabsorb bicarbonate, which leads to a disturbance in the acid-base balance of the body, resulting in metabolic acidosis with a normal anion gap.

Infants with RTA typically exhibit clinical features such as failure to thrive, growth retardation, and may also present with electrolyte imbalances. The normal anion gap acidosis is indicative of a primary defect in the renal tubules rather than loss of bicarbonate due to gastrointestinal losses (as seen in chronic diarrhea), or another systemic metabolic disorder such as a urea cycle defect.

The character of the acidosis and the accompanying symptoms seen in the infant strongly suggest RTA as the underlying etiology. This situation can arise from various causes, including inherited metabolic disorders or acquired conditions, but the key factor is the disturbance occurring primarily in the renal tubules. This makes RTA the most appropriate diagnosis in this context.