An asymptomatic adult undergoing routine urine examination is found to have a persistently positive reducing substance test. Confirmatory testing identifies large amounts of fructose in urine. Blood glucose is normal, there is no hypoglycemia after sucrose load, and the liver enzymes are normal. The most likely enzyme defect is:
- A Aldolase B in the liver
- B Fructose transporter GLUT5 in the intestine
- C Fructokinase in the liver and kidney ✓
- D Fructose-1-phosphate aldolase cleavage in the renal tubule
Correct answer: C. Fructokinase in the liver and kidney
Explanation
Essential fructosuria is a benign autosomal recessive condition caused by fructokinase deficiency. Fructose cannot be phosphorylated to fructose-1-phosphate, so it appears unchanged in urine and gives a positive reducing substance test, but there is no metabolic trapping, no hypoglycemia, and no liver injury. Aldolase C deficiency causes hereditary fructose intolerance, which produces vomiting, hypoglycemia, and hepatomegaly after fructose exposure.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.