Which type of renal tubular acidosis is characterized by abnormal HCO3 and rickets?

Type II renal tubular acidosis (RTA), also known as bicarbonate wasting or proximal RTA, is characterized by a defect in the proximal tubular reabsorption of bicarbonate (HCO3). This leads to abnormal bicarbonate levels in the blood, resulting in metabolic acidosis, as the kidneys cannot adequately reabsorb bicarbonate back into the bloodstream.

One of the key clinical associations of Type II RTA is rickets, which is particularly important in the context of childhood. The proximal tubular dysfunction can also lead to impaired phosphate reabsorption, which contributes to the development of rickets due to decreased phosphate availability for bone mineralization. Additionally, rickets can also result from calcium metabolism issues linked with renal tubular disorders.

Since Type II RTA results in the loss of bicarbonate, patients may present with a lower serum bicarbonate level. Over time, if the renal tubular function is severely impaired, the serum bicarbonate level may reach a point where it approaches the lower limit of normal.

In summary, the combination of abnormal bicarbonate levels and the association with rickets specifically points to Type II RTA as the correct answer.