Urine screening of an apparently healthy pregnant woman demonstrates a positive Clinitest reaction. However, blood glucose levels were within normal limits, and more specific testing for urine glucose is negative. The woman has been unaware of any metabolic problems and has been living a normal life. Deficiency of which of the following enzymes would most likely produce this presentation?

Glucose, galactose, and fructose are all reducing sugars, and elevations of all of these sugars can be detected with Clinitest. Neither lactose nor pyruvate can be detected, thus eliminating lactase and pyruvate dehydrogenase as plausible choices. This leaves three possibilities: fructokinase, fructose 1-phosphate aldolase, and galactose 1-P-uridyltransferase. Of these, only fructokinase deficiency produces a mild (usually completely asymptomatic) condition known as fructosuria. Fructose 1-phosphate aldolase deficiency produces severe hereditary fructose intolerance. Galactose 1-P-uridyl transferase deficiency produces classic galactosemia. Lactase deficiency produces lactose intolerance. Ref: Bender D.A., Mayes P.A. (2011). Chapter 21. The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.