Acid-base balance: 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 balance

Sample item

A 4-week-old infant with persistent vomiting from pyloric stenosis has arterial blood gas results pH 7.52, PaCO2 48 mmHg and HCO3 38 mmol/L. Which interpretation is correct?

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

The answer and the reasoning

A. Metabolic alkalosis with partial respiratory compensation

A high pH with a high bicarbonate indicates metabolic alkalosis, here from loss of gastric hydrochloric acid. The raised PaCO2 reflects hypoventilation as the respiratory compensation, which cannot fully correct the pH.

Option by option

  • A. Metabolic alkalosis with partial respiratory compensation (correct)

    Correct. High pH with high bicarbonate is primary metabolic alkalosis, and the raised PaCO2 is the compensation.

  • B. Respiratory alkalosis with renal compensation

    Respiratory alkalosis would show a low PaCO2, not a high one.

  • C. Metabolic acidosis with respiratory compensation

    Metabolic acidosis would give a low pH and low bicarbonate, the opposite of these values.

  • D. Respiratory acidosis with metabolic compensation

    Respiratory acidosis would give a low pH; here pH is high and bicarbonate is the primary change.

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 A, Metabolic alkalosis with partial respiratory compensation. A high pH with a high bicarbonate indicates metabolic alkalosis, here from loss of gastric hydrochloric acid.

Why is "Respiratory alkalosis with renal compensation" not the answer?

Respiratory alkalosis would show a low PaCO2, not a high one.

Why is "Metabolic acidosis with respiratory compensation" not the answer?

Metabolic acidosis would give a low pH and low bicarbonate, the opposite of these values.

Why is "Respiratory acidosis with metabolic compensation" not the answer?

Respiratory acidosis would give a low pH; here pH is high and bicarbonate is the primary change.

What concept does this question test?

It tests Acid-base balance in Physiology. The raised PaCO2 reflects hypoventilation as the respiratory compensation, which cannot fully correct the pH.

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 balance?

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.