the proximal tubular cells called Fanconi syndrome, in which there is also phosphaturia, glycosuria, aminoaciduria, uricosuria, and tubular proteinuria. The principal feature of Fanconi syndrome is bone demineralization (osteomalacia or rickets) due to phosphate wasting. Type 3: combined proximal and distal In some patients, RTA shares features of both dRTA and pRTA. This rare pattern was observed in the 1960s and 1970s as a transient phenomenon in infants and children with dRTA (possibly in relation with some exogenous factor such as high salt intake) and is no longer observed. This form of RTA has also been referred to as juvenile RTA. Combined dRTA and pRTA is also observed as the result of inherited carbonic anhydrase II deficiency. Mutations in the gene encoding this enzyme give rise to an autosomal recessive syndrome of osteopetrosis, renal tubular acidosis, cerebral calcification, and mental retardation. It is very rare and cases from all over the world have been reported, of which about 70% are from the Maghreb region of North Africa, possibly due to the high prevalence of consanguinity there. The kidney problems are treated as described above. There is no treatment for the osteopetrosis or cerebral calcification. Type 3 is rarely discussed. Most comparisons of RTA are limited to a comparison of types 1, 2, and 4. Type 4: absolute hypoaldosteronism or aldosterone insensitivity Type 4 RTA is not actually a tubular disorder at all nor does it have a clinical syndrome similar to the other types of RTA described above. It was included in the classification of renal tubular acidoses as it is associated with a mild (normal anion gap) metabolic acidosis due to a physiological reduction in proximal tubular ammonium excretion (impaired ammoniagenesis), which is secondary to hypoaldosteronism, and results in a decrease in urine buffering capacity. Its cardinal feature is hyperkalemia, and measured urinary acidification is normal, hence it is often called hyperkalemic RTA or tubular hyperkalemia. Causes include: Aldosterone deficiency (hypoaldosteronism): Primary vs. hyporeninemic (including diabetic nephropathy) Aldosterone resistance Drugs: NSAIDs, ACE inhibitors and ARBs, Eplerenone, Spironolactone, Trimethoprim, Pentamidine Pseudohypoaldosteronism History Renal tubular acidosis was first described in 1935 by Lightwood and 1936 by Butler et al. in children. Baines et al. first described it in adults in 1945. Donald L. Lewis postulated the character Tiny Tim, of A Christmas Carol, was suffering from renal tubular acidosis. Researchers published in PLOS ONE in 2009 speculated that the infamously afflicted