Metabolism: adaptation to starvation: 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 · Metabolism: adaptation to starvation

Sample item

A man on a hunger strike has been fasting for 10 days. His urinary nitrogen loss has fallen compared with day 2. What best explains the fall?

AThe brain oxidises free fatty acids directlyBRed cells have switched to ketone body oxidationCThe brain now derives most of its energy from ketone bodies, reducing the glucose needed from muscle amino acidsDCortisol has fallen and spared protein

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

The answer and the reasoning

C. The brain now derives most of its energy from ketone bodies, reducing the glucose needed from muscle amino acids

After about 3 days the liver produces large amounts of ketone bodies, and the brain adapts to use them for much of its energy, cutting its glucose requirement from roughly 120 g to about 40 g daily. This reduces gluconeogenesis from amino acids and spares muscle protein. Fatty acids do not cross the blood-brain barrier appreciably.

Option by option

A. The brain oxidises free fatty acids directly
Albumin-bound fatty acids cannot supply the brain directly.
B. Red cells have switched to ketone body oxidation
Red cells lack mitochondria and cannot oxidise ketone bodies.
C. The brain now derives most of its energy from ketone bodies, reducing the glucose needed from muscle amino acids (correct)
Correct. Ketone adaptation lowers the demand for gluconeogenic amino acids.
D. Cortisol has fallen and spared protein
Cortisol tends to stay raised in fasting and does not explain nitrogen sparing.

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 C, The brain now derives most of its energy from ketone bodies, reducing the glucose needed from muscle amino acids. After about 3 days the liver produces large amounts of ketone bodies, and the brain adapts to use them for much of its energy, cutting its glucose requirement from roughly 120 g to about 40 g daily.

Why is "The brain oxidises free fatty acids directly" not the answer?

Albumin-bound fatty acids cannot supply the brain directly.

Why is "Red cells have switched to ketone body oxidation" not the answer?

Red cells lack mitochondria and cannot oxidise ketone bodies.

Why is "Cortisol has fallen and spared protein" not the answer?

Cortisol tends to stay raised in fasting and does not explain nitrogen sparing.

What concept does this question test?

It tests Metabolism: adaptation to starvation in Biochemistry. This reduces gluconeogenesis from amino acids and spares muscle protein. Fatty acids do not cross the blood-brain barrier appreciably.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Metabolism: adaptation to starvation?

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.