Amino acid metabolism: ammonia management: 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 · Amino acid metabolism: ammonia management

Sample item

A man with cirrhosis has asterixis and confusion with a high arterial ammonia. He is started on oral lactulose. Which mechanism explains its benefit?

AIt conjugates with ammonia in plasma for renal excretionBColonic bacteria convert it to acids, trapping ammonia as ammonium and speeding its faecal lossCIt activates carbamoyl phosphate synthetase I in hepatocytesDIt is absorbed and converted to glutamine in skeletal muscle

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

The answer and the reasoning

B. Colonic bacteria convert it to acids, trapping ammonia as ammonium and speeding its faecal loss

Lactulose is not absorbed in the small intestine and is fermented by colonic bacteria to lactic and acetic acids. The lowered colonic pH converts diffusible ammonia into non-absorbable ammonium, and the osmotic laxative effect increases faecal excretion. Rifaximin is added to reduce ammonia-producing bacteria.

Option by option

A. It conjugates with ammonia in plasma for renal excretion
Lactulose does not bind ammonia in plasma; sodium benzoate and phenylacetate act by conjugation.
B. Colonic bacteria convert it to acids, trapping ammonia as ammonium and speeding its faecal loss (correct)
Correct. Acidification of the colon traps ammonia as ammonium and the laxative effect removes it.
C. It activates carbamoyl phosphate synthetase I in hepatocytes
Lactulose has no activating effect on the urea cycle, and the cirrhotic liver is the failing organ.
D. It is absorbed and converted to glutamine in skeletal muscle
Lactulose is a non-absorbable disaccharide and is not converted to glutamine in muscle.

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 B, Colonic bacteria convert it to acids, trapping ammonia as ammonium and speeding its faecal loss. Lactulose is not absorbed in the small intestine and is fermented by colonic bacteria to lactic and acetic acids.

Why is "It conjugates with ammonia in plasma for renal excretion" not the answer?

Lactulose does not bind ammonia in plasma; sodium benzoate and phenylacetate act by conjugation.

Why is "It activates carbamoyl phosphate synthetase I in hepatocytes" not the answer?

Lactulose has no activating effect on the urea cycle, and the cirrhotic liver is the failing organ.

Why is "It is absorbed and converted to glutamine in skeletal muscle" not the answer?

Lactulose is a non-absorbable disaccharide and is not converted to glutamine in muscle.

What concept does this question test?

It tests Amino acid metabolism: ammonia management in Biochemistry. The lowered colonic pH converts diffusible ammonia into non-absorbable ammonium, and the osmotic laxative effect increases faecal excretion. Rifaximin is added to reduce ammonia-producing bacteria.

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 Amino acid metabolism: ammonia management?

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.