Clinical biochemistry: compensation in chronic respiratory acidosis: 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 · Clinical biochemistry: compensation in chronic respiratory acidosis

Sample item

A man with long-standing COPD has stable chronic CO2 retention with PaCO2 of 70 mmHg. Which serum bicarbonate is best explained by full renal compensation of this chronic disorder?

AAbout 18 mmol/LBAbout 35 mmol/LCAbout 27 mmol/LDAbout 24 mmol/L

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

The answer and the reasoning

B. About 35 mmol/L

In chronic respiratory acidosis, bicarbonate rises by about 3.5 to 4 mmol/L for each 10 mmHg rise in PaCO2, so a PaCO2 of 70 gives about 34 to 36 mmol/L. In acute respiratory acidosis the rise is only about 1 mmol/L per 10 mmHg. A value of 18 would mean a superimposed metabolic acidosis.

Option by option

A. About 18 mmol/L
A low bicarbonate would indicate an additional metabolic acidosis.
B. About 35 mmol/L (correct)
Correct. Renal retention raises bicarbonate by roughly 3.5 to 4 per 10 mmHg of CO2 in chronic disease.
C. About 27 mmol/L
This rise of about 1 per 10 mmHg applies to acute respiratory acidosis.
D. About 24 mmol/L
A normal bicarbonate means no renal compensation has occurred.

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, About 35 mmol/L. In chronic respiratory acidosis, bicarbonate rises by about 3.

Why is "About 18 mmol/L" not the answer?

A low bicarbonate would indicate an additional metabolic acidosis.

Why is "About 27 mmol/L" not the answer?

This rise of about 1 per 10 mmHg applies to acute respiratory acidosis.

Why is "About 24 mmol/L" not the answer?

A normal bicarbonate means no renal compensation has occurred.

What concept does this question test?

It tests Clinical biochemistry: compensation in chronic respiratory acidosis in Biochemistry. 5 to 4 mmol/L for each 10 mmHg rise in PaCO2, so a PaCO2 of 70 gives about 34 to 36 mmol/L. In acute respiratory acidosis the rise is only about 1 mmol/L per 10 mmHg. A value of 18 would mean a superimposed metabolic acidosis.

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 Clinical biochemistry: compensation in chronic respiratory acidosis?

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.