Carbohydrate metabolism: fructose and lipogenesis: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Carbohydrate metabolism: fructose and lipogenesis

Sample item

A 30-year-old man consuming large amounts of sugar-sweetened beverages develops raised hepatic triglyceride and fatty liver. Which feature of hepatic fructose metabolism makes it more lipogenic than glucose?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Entry past phosphofructokinase-1 floods the pathway with trioses

Fructokinase and aldolase B convert fructose to triose phosphates, bypassing the regulated phosphofructokinase-1 step. The resulting unrestricted supply of acetyl-CoA and glycerol-3-phosphate promotes fatty acid synthesis and triglyceride formation, and VLDL output rises.

Option by option

  • A. Fructose is phosphorylated by glucokinase and stored as glycogen

    Fructose is phosphorylated by fructokinase, not glucokinase, and much of it is channelled to lipid rather than glycogen.

  • B. Fructose inhibits acetyl-CoA carboxylase directly

    Acetyl-CoA carboxylase is stimulated by citrate, not inhibited by fructose, in the lipogenic state.

  • C. Fructose stimulates glucagon release from the pancreas

    Fructose does not stimulate glucagon; high fructose flux favours lipogenesis, which is an insulin-like direction.

  • D. Entry past phosphofructokinase-1 floods the pathway with trioses (correct)

    Correct. Fructose bypasses PFK-1 control, delivering unregulated carbon for fatty acid and triglyceride synthesis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Entry past phosphofructokinase-1 floods the pathway with trioses. Fructokinase and aldolase B convert fructose to triose phosphates, bypassing the regulated phosphofructokinase-1 step.

Why is "Fructose is phosphorylated by glucokinase and stored as glycogen" not the answer?

Fructose is phosphorylated by fructokinase, not glucokinase, and much of it is channelled to lipid rather than glycogen.

Why is "Fructose inhibits acetyl-CoA carboxylase directly" not the answer?

Acetyl-CoA carboxylase is stimulated by citrate, not inhibited by fructose, in the lipogenic state.

Why is "Fructose stimulates glucagon release from the pancreas" not the answer?

Fructose does not stimulate glucagon; high fructose flux favours lipogenesis, which is an insulin-like direction.

What concept does this question test?

It tests Carbohydrate metabolism: fructose and lipogenesis in Biochemistry. The resulting unrestricted supply of acetyl-CoA and glycerol-3-phosphate promotes fatty acid synthesis and triglyceride formation, and VLDL output rises.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Carbohydrate metabolism: fructose and lipogenesis?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.