Renal and acid-base: titratable acid: 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.

Try it

Physiology · Renal and acid-base: titratable acid

Sample item

When urine is titrated back to the pH of plasma (7.4) with alkali, the amount of alkali needed is the titratable acidity. Which urinary buffer accounts for most of it?

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

The answer and the reasoning

A. Monohydrogen phosphate

Hydrogen ions secreted into the tubule are buffered by filtered phosphate (HPO4 2- to H2PO4-), whose pK of 6.8 lies close to the tubular pH range. This buffering is measured as titratable acid. Ammonia has a pK of about 9.2 and traps H+ as ammonium, which is not measured by titration to pH 7.4.

Option by option

  • A. Monohydrogen phosphate (correct)

    Correct. Phosphate is the main titratable buffer.

  • B. Sulphate

    Sulphate is a poor buffer in the physiological pH range.

  • C. Ammonia

    Ammonium is not counted in titratable acid, since its pK is high and it is excreted separately.

  • D. Bicarbonate

    Bicarbonate is largely reabsorbed and is absent from acidic urine.

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 Physiology questions

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Monohydrogen phosphate. Hydrogen ions secreted into the tubule are buffered by filtered phosphate (HPO4 2- to H2PO4-), whose pK of 6.

Why is "Sulphate" not the answer?

Sulphate is a poor buffer in the physiological pH range.

Why is "Ammonia" not the answer?

Ammonium is not counted in titratable acid, since its pK is high and it is excreted separately.

Why is "Bicarbonate" not the answer?

Bicarbonate is largely reabsorbed and is absent from acidic urine.

What concept does this question test?

It tests Renal and acid-base: titratable acid in Physiology. 8 lies close to the tubular pH range. This buffering is measured as titratable acid. Ammonia has a pK of about 9.2 and traps H+ as ammonium, which is not measured by titration to pH 7.4.

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 Renal and acid-base: titratable acid?

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.