To use lactose as fuel, the body must first break it down into two simpler sugars: glucose and galactose.
Then, if the tests show an excess of galactose (milk sugar) in the blood or urine, doctors know what the trouble is and how to remedy it.
Because of a defect in the genes in the nuclei of his cells, the victim was unable to produce the essential enzyme that enables the body to metabolize galactose, a simple sugar .