American Board of Internal Medicine (ABIM) Certification Practice Exam

In Fanconi syndrome, what type of renal compensation occurs due to bicarbonate loss?

1. Type 1 renal tubular acidosis

2. Type 2 renal tubular acidosis

3. Type 3 renal tubular acidosis

4. Acute kidney injury

The correct answer is: Type 2 renal tubular acidosis

In Fanconi syndrome, there is a generalized dysfunction of the proximal renal tubule, leading to a variety of metabolic abnormalities, including the loss of bicarbonate. The loss of bicarbonate results in a decreased serum bicarbonate level, leading to a metabolic acidosis. This condition is classified under Type 2 renal tubular acidosis (RTA), which specifically is characterized by the inability of the proximal tubule to reabsorb bicarbonate effectively. In Type 2 RTA, bicarbonate is not adequately reabsorbed from the urine, especially at higher urinary pH levels, which corresponds to the pathophysiology seen in Fanconi syndrome. As the bicarbonate concentration in the serum drops, the body compensates by undergoing renal mechanisms in an attempt to restore bicarbonate levels. This type of acidosis primarily occurs in contexts where proximal tubular function is impaired, reflecting the condition's association with other solute losses, such as phosphate and glucose, along with bicarbonate. Other types of renal tubular acidosis and acute kidney injury differ in their underlying mechanisms and the specific tubular dysfunctions involved, which do not align with the bicarbonate handling issues seen in Fanconi syndrome. Thus, the presence of bicarbonate loss as a defining characteristic makes Type