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The glomeruli filter approximately 150 liters of ultrafiltrate
daily that are delivered to the renal tubules. The renal tubules
reabsorb organic solutes, salt, and water to maintain a constant
extracellular fluid volume and composition. In addition, organic
anions and cations, which are protein bound and not filtered by
the glomerulus, are secreted by the proximal tubule. The final urine
contains about one hundredth of the volume and sodium as that in
the original glomerular filtrate, but contains all waste products. There
are 12 nephron segments that have different transport properties
to perform this task. Disorders of tubular function can be due to
inherited defects in transporters or mutations in factors that regulate
transport, or result from inherited or acquired disorders that cause
tubular injury. Renal transport disorders can be mild with little
to no clinical consequences, or life threatening, depending on the
transporters and nephron segments affected.
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Fanconi syndrome is a generalized proximal tubule transport disorder.1 The
proximal tubule is responsible for the reabsorption of all filtered
glucose and amino acids, and 80% of the filtered bicarbonate and
phosphate. Patients with Fanconi syndrome have hypophosphatemia,
hypokalemia, and hyperchloremic metabolic acidosis.
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The luminal fluid entering the proximal tubule is an ultrafiltrate
of plasma. Most solutes are transported across the apical membrane
in conjunction with sodium. The driving force for solute transport
is the low intracellular sodium generated by the basolateral Na+-K+-ATPase.
A generalized decrease in proximal tubular transport could result
from a primary injury to the Na+-K+-ATPase
pump or a decrease in intracellular adenosine triphosphate (ATP)
that fuels the pump. Theoretically, an increase in the permeability
of the proximal tubule paracellular pathway could result in Fanconi
syndrome, but this theory has been discounted. Although the cellular
basis for most causes of Fanconi syndrome has not been determined,
most studies have demonstrated that the proximal tubule transport
defect in Fanconi syndrome is the result of a decrease in intracellular
ATP.
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Differential Diagnosis
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The causes for Fanconi syndrome are listed in Table
474-1. Diagnosis of Fanconi syndrome is
usually made due to the presence of symptoms, including failure
to thrive, severe rickets, and sometimes bouts of polydipsia, polyuria,
and dehydration. Laboratory findings include hypophosphatemia, hypokalemia,
and acidosis.2 Evaluation of the urine reveals glucosuria,
generalized aminoaciduria, and hyperphosphaturia, despite the hypophosphatemia,
a stimulus that increases proximal tubule phosphate absorption.
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