Acid-base: Henderson-Hasselbalch: practice question

A Physiology 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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Physiology · Acid-base: Henderson-Hasselbalch

Sample item

An arterial sample shows bicarbonate 12 mmol/L and PCO2 40 mmHg. Using pH = 6.1 + log [HCO3-] / (0.03 x PCO2), what is the pH?

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

The answer and the reasoning

C. 7.10

Dissolved carbon dioxide is 0.03 x 40 = 1.2 mmol/L. The ratio 12/1.2 equals 10, and log 10 is 1. So pH = 6.1 + 1 = 7.10. A normal ratio is 24/1.2 = 20, whose log is 1.3, giving 7.4.

Option by option

  • A. 7.25

    The calculation does not give this value; log 10 equals exactly 1.

  • B. 6.90

    6.90 would need a ratio of about 6.3, which means bicarbonate of about 7.6 at this PCO2.

  • C. 7.10 (correct)

    Correct. 6.1 + log (12/1.2) = 7.10.

  • D. 7.40

    A pH of 7.40 needs a bicarbonate of 24 at this PCO2.

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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, 7.10. Dissolved carbon dioxide is 0.

Why is "7.25" not the answer?

The calculation does not give this value; log 10 equals exactly 1.

Why is "6.90" not the answer?

6.90 would need a ratio of about 6.3, which means bicarbonate of about 7.6 at this PCO2.

Why is "7.40" not the answer?

A pH of 7.40 needs a bicarbonate of 24 at this PCO2.

What concept does this question test?

It tests Acid-base: Henderson-Hasselbalch in Physiology. 03 x 40 = 1.2 mmol/L. The ratio 12/1.2 equals 10, and log 10 is 1. So pH = 6.1 + 1 = 7.10. A normal ratio is 24/1.2 = 20, whose log is 1.3, giving 7.4.

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 Acid-base: Henderson-Hasselbalch?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.