A 30 year old male was found to have excessive urinary glucose levels. However, his blood glucose levels were normal. Further investigations revealed high levels of only L-xylulose in urine. A genetic defect in which of the following pathway is most likely?

Essential Pentosuria defect in Uronic Acid Pathway: Pentosuria is a condition where the sugar xylitol, a pentose, presents in the urine in unusually high concentrations. It was characterized as an inborn error of carbohydrate metabolism in 1908. It is associated with a deficiency of L-xylulose reductase, necessary for xylitol metabolism. L-Xylulose is a reducing sugar, so it may give false diagnosis of diabetes, as it is found in high concentrations in urine. However, glucose metabolism is normal in people with pentosuria and they are not diabetic. Patients of pentosuria have a low concentration of the sugar d-xyloketose. Using phenyl pentosazone crystals, phloroglucin reaction, and absorption spectrum, pentose can be traced back as the reducing substance in urine, with those that have pentosuria.   Key Concept:  L-Xylulose is a reducing sugar, so it may give false diagnosis of diabetes, as it is found in high concentrations in urine.