The first step in fructose metabolism in the liver is
**Core Concept**
Fructose metabolism in the liver involves its conversion into intermediates that can be fed into the glycolytic pathway or the citric acid cycle. The initial step in fructose metabolism is crucial for its further breakdown and utilization.
**Why the Correct Answer is Right**
The first step in fructose metabolism in the liver is its phosphorylation to fructose-1-phosphate by the enzyme fructokinase. This is a unique step that distinguishes fructose metabolism from glucose metabolism, which directly enters the glycolytic pathway. Fructose-1-phosphate is then split into glyceraldehyde and dihydroxyacetone phosphate by the enzyme aldolase B, which is specific to the liver.
**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate dehydrogenase is an enzyme involved in the pentose phosphate pathway, not in the initial step of fructose metabolism.
**Option B:** Pyruvate kinase is an enzyme involved in the final step of glycolysis, not in the initial step of fructose metabolism.
**Option C:** HMG-CoA synthase is an enzyme involved in the biosynthesis of cholesterol, not in the metabolism of fructose.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that fructose intolerance can lead to an accumulation of fructose-1-phosphate, which can cause damage to the liver and kidneys. This condition is known as hereditary fructose intolerance (HFI), and it's caused by a deficiency of the enzyme aldolase B.
**Correct Answer:** C. Fructokinase.