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DOI:10.1097/MOP.0000000000000461 I would like to introduce this year’s nephrology section of Current Opinion in Pediatrics with a case that I saw many years ago. The patient was a 15-yearold female who had a 5-year history of malaise, weakness and constipation. For these complaints, she had been evaluated by her pediatrician who thought that she had a mood disorder and referred her to a psychiatrist. After years of psychiatric evaluation and therapy, she was no better. She was sent to a gastroenterologist at my institution who saw her for constipation and drew laboratory studies revealing a hypokalemic metabolic alkalosis. There was no history of vomiting or diarrhea. The gastroenterologist referred the patient to me. I asked her if she liked pickles and she said that she loved pickles. I then ask her if she drank the juice. She burst into tears, turned to her mother and said that she finally found the doctor that realized that she is not crazy. The diagnosis will be apparent later in this discussion, but the point that I am trying to make is that tubular disorders are not as rare as you might think, and they are often misdiagnosed because of constitutional symptoms. In this series of review articles, we are going to work our way down the nephron starting with the proximal tubule. We are not going to be able to discuss all of the inherited defects in tubular transport but will focus on some of the more common ones. I will first discuss the organization of the nephron and the reabsorptive capacity of each segment. The kidney filters 120 ml/min/1.73 m or 150–170 l of fluid a day. The tubules reabsorb 99% of this fluid and excrete about 1.5 l of urine a day. If there was no tubular reabsorption, we would be very volume depleted in minutes and become dust in a few hours. The proximal tubule reabsorbs two-thirds of the filtered sodium and volume in an isotonic fashion. The proximal tubule reabsorbs all of the filtered glucose and amino acids, which will be absorbed as there are no transporters for these solutes at distal sites. The proximal tubule also is responsible for the reabsorption of 90% of the filtered phosphate, 80% of the filtered bicarbonate and about two-thirds of the filtered chloride. Organic anion and cation secretion also occurs in the proximal tubule. The thick ascending limb reabsorbs 25% of the filtered