Acid-base chemistry: 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 · Acid-base chemistry

Sample item

A patient has plasma bicarbonate of 24 mmol/L and PaCO2 of 80 mmHg. Take pKa as 6.1, CO2 solubility as 0.03 mmol/L per mmHg, and log 10 = 1. What is the approximate pH?

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

The answer and the reasoning

A. 7.1

Dissolved CO2 is 0.03 times 80, which is 2.4 mmol/L. The bicarbonate to CO2 ratio is 24 divided by 2.4, which is 10, and its log is 1. So the pH is 6.1 plus 1, which is 7.1, showing respiratory acidosis.

Option by option

  • A. 7.1 (correct)

    Correct. The ratio is 10, so pH equals 6.1 plus 1.

  • B. 6.8

    A pH of 6.8 would need a ratio near 5, which means a PaCO2 of about 160 mmHg.

  • C. 7.4

    A pH of 7.4 needs a ratio of 20, which corresponds to a PaCO2 of about 40 mmHg.

  • D. 7.7

    A pH near 7.7 needs a ratio near 40, which would need a PaCO2 of about 20 mmHg.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, 7.1. Dissolved CO2 is 0.

Why is "6.8" not the answer?

A pH of 6.8 would need a ratio near 5, which means a PaCO2 of about 160 mmHg.

Why is "7.4" not the answer?

A pH of 7.4 needs a ratio of 20, which corresponds to a PaCO2 of about 40 mmHg.

Why is "7.7" not the answer?

A pH near 7.7 needs a ratio near 40, which would need a PaCO2 of about 20 mmHg.

What concept does this question test?

It tests Acid-base chemistry in Biochemistry. 03 times 80, which is 2.4 mmol/L. The bicarbonate to CO2 ratio is 24 divided by 2.4, which is 10, and its log is 1. So the pH is 6.1 plus 1, which is 7.1, showing respiratory 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 Acid-base chemistry?

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.