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Galactokinase deficiency = mere deficiency of galactokinase = milder presentation
Classic galactosemia = deficiency of galactose-1-phosphate uridyltransferase (GALT) = severe presentation
Galactokinase deficiency will show up in questions as infantile cataracts and galactose in the urine. That’s it. They’ll give you an infant who recently started breastfeeding who now has problems seeing or who has leukocoria.
The USMLE likes to say that “a reducing sugar” was detected in the urine.
To that effect, in fructose disorders, they’ll say the same thing. Galactose and fructose are not detected on urine dipstick because that test is specific for glucose, but they are still detected as reducing sugar in the urine.
| Urine dipstick is specific for glucose and won’t detect galactose or fructose. But the USMLE will say “reducing sugars in the urine” for galactose (and fructose) disorders. |
Rarely the question might mention that the infant doesn’t develop social smile or track objects, so be aware that mild cognitive delay can be seen, but in general, visual problems + reducing sugar in the urine in an otherwise benign presentation = galactokinase deficiency.
In contrast, classic galactosemia will prevent severely.
Look for hepatosplenomegaly, jaundice, and mental impairment.
| ↓ vision + reducing sugar in urine. Does the child have jaundice + HSM + severe mental impairment?
No, there’s no jaundice or HSM. There’s no (or mild) mental impairment. → Galactokinase deficiency Yes, there’s jaundice, HSM, and severe mental impairment. → Classic galactosemia |
USMLE questions wanting you to distinguish between galactokinase deficiency and classic galactosemia will always give you hepatosplenomegaly and jaundice if the answer is classic galactosemia. Mental impairment can be seen in galactokinase deficiency, but if the question does illustrate developmental delay, it will always be severe in classic galactosemia, whereas it will only be mild in galactokinase deficiency.
| For the galactose (and fructose) disorders, deficiency of the more downstream (second) enzyme causes the severe presentation. This is because of phosphate trapping in the liver. Buildup of galactose-1-phosphate means more phosphate is tied up in the form of that intermediate. |
The child must stop breastfeeding if a galactose disorder is diagnosed because lactose in breast milk is catabolized to glucose + galactose.
| It is exceedingly HY that galactitol causes the cataracts. Aldose reductase turns galactose into galactitol. |
USMLE questions are obsessed with aldose reductase being the cause of cataracts in galactose disorders. Aldose reductase converts galactose to galactitol, which, like sorbitol in diabetes, becomes trapped in the lens and causes damage.
There’s also one last really important factoid about classic galactosemia:
| E. coli sepsis in a neonate is associated with classic galactosemia.
Why? Who knows. We can speculate it has something to do with E. coli being a gram (-) lactose-fermenting rod, and lactose = glucose + galactose. In addition, infections with Group B Strep (S. agalactiae), E. coli, and Listeria are common in neonates. |
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