Carbohydrate metabolism: GLUT1 deficiency syndrome: 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: GLUT1 deficiency syndrome

Sample item

An infant has seizures from the second month, acquired microcephaly and developmental delay. Blood glucose is normal, but CSF glucose is low with a normal CSF lactate. Which diagnosis is most likely?

AGLUT1 deficiency syndromeBPyruvate dehydrogenase deficiencyCGlycogen synthase deficiencyDHyperinsulinaemic hypoglycaemia

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

The answer and the reasoning

A. GLUT1 deficiency syndrome

GLUT1 carries glucose across the blood-brain barrier and into astrocytes, so its deficiency lowers CSF glucose while blood glucose is normal. The brain is starved of glucose, and a ketogenic diet is the treatment because ketone bodies enter the brain by a different transporter. Lactate is not raised, which separates it from pyruvate dehydrogenase deficiency.

Option by option

A. GLUT1 deficiency syndrome (correct)
Correct. Impaired glucose transport across the blood-brain barrier gives low CSF glucose with normal blood glucose.
B. Pyruvate dehydrogenase deficiency
Pyruvate dehydrogenase deficiency raises lactate in blood and CSF.
C. Glycogen synthase deficiency
Glycogen synthase deficiency causes fasting hypoglycaemia in blood and does not lower CSF glucose alone.
D. Hyperinsulinaemic hypoglycaemia
Hyperinsulinaemic hypoglycaemia lowers blood glucose, which is normal here.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, GLUT1 deficiency syndrome. GLUT1 carries glucose across the blood-brain barrier and into astrocytes, so its deficiency lowers CSF glucose while blood glucose is normal.

Why is "Pyruvate dehydrogenase deficiency" not the answer?

Pyruvate dehydrogenase deficiency raises lactate in blood and CSF.

Why is "Glycogen synthase deficiency" not the answer?

Glycogen synthase deficiency causes fasting hypoglycaemia in blood and does not lower CSF glucose alone.

Why is "Hyperinsulinaemic hypoglycaemia" not the answer?

Hyperinsulinaemic hypoglycaemia lowers blood glucose, which is normal here.

What concept does this question test?

It tests Carbohydrate metabolism: GLUT1 deficiency syndrome in Biochemistry. The brain is starved of glucose, and a ketogenic diet is the treatment because ketone bodies enter the brain by a different transporter. Lactate is not raised, which separates it from pyruvate dehydrogenase deficiency.

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: GLUT1 deficiency syndrome?

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.