A 6-month-old boy is brought in by his mother to the pediatrician for evaluation of new onset vomiting. The mother states that the infant is vomiting more than usual for the past 2 weeks, ever since she introduced apple juice to his diet. The infant was born healthy and at term with no complications. The mother had routine prenatal care throughout the pregnancy. Physical examination reveals an irritable infant with diffuse jaundice. Laboratory tests are unremarkable with the exception of hypoglycemia.
Which of the following enzymes is most likely affected?
- A) Aldolase B
- B) Fructokinase
- C) Galactokinase
- D) Galactose-1-phosphate uridyltransferase
- E) Lactase
A) Aldolase B
This infant with recurrent vomiting and jaundice occurring when apple juice was introduced to his diet has hereditary fructose Intolerance, which is caused by a deficiency in Aldolase B. This deficiency causes fructose-1-phosphate to accumulate, resulting in a decrease in available phosphate, leading to inhibition of gluconeogenesis and glycogenolysis. Other foods that have fructose include fruits and honey. Treatment is avoidance of fructose, sucrose, and sorbitol. If not avoided, it may lead to cirrhosis of the liver.
Answer choice B: Fructokinase, is incorrect. Deficiency in fructokinase causes essential fructosuria which is a benign asymptomatic condition. Fructose will appear in the blood and urine.
Answer choice C: Galactokinase, is incorrect. Deficiency in galactokinase is also a benign condition; however, symptoms may develop such as failure to track objects or failure to develop a social smile.
Answer choice D: Galactose-1-phosphate uridyltransferase, is incorrect. Deficiency of galactose-1-phosphate uridyltransferase causes classic galactosemia. This causes an accumulation of galactitol. Symptoms develop as soon as feeding occurs in infants. Symptoms include failure to thrive, infantile cataracts, hepatomegaly, and intellectual disability.
Answer choice E: Lactase, is incorrect. Deficiency in lactase causes lactose intolerance. Symptoms include bloating, cramps, flatulence, and osmotic diarrhea.
Key Learning Point
Aldolase B deficiency leads to hereditary fructose intolerance, an inborn error of fructose metabolism. Affected individuals are asymptomatic until they ingest fructose, sucrose, or sorbitol. The enzymatic block at aldolase B causes an accumulation of fructose-1-phosphate, which has downstream effects on gluconeogenesis and regeneration of adenosine triphosphate (ATP). Symptoms include irritability, vomiting, poor feeding, hypoglycemia, jaundice, hemorrhage, and convulsions.