Blood and immunity: osmotic fragility: 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 · Blood and immunity: osmotic fragility

Sample item

A 12-year-old has jaundice, splenomegaly and spherocytes on the smear, with a negative direct antiglobulin test. When his red cells are placed in graded hypotonic saline, how do they behave compared with normal cells?

AThey behave exactly like normal cells in salineBThey resist lysis in even distilled waterCThey lyse at a higher saline concentration than normal cellsDThey lyse at a lower saline concentration than normal cells

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

The answer and the reasoning

C. They lyse at a higher saline concentration than normal cells

Spherocytes have lost membrane surface area, for example from defects of spectrin or ankyrin, so they have little reserve to swell. They burst after taking in only a small amount of water, which means increased osmotic fragility. Target cells and thalassaemic cells show the opposite, decreased fragility.

Option by option

A. They behave exactly like normal cells in saline
The test is abnormal in hereditary spherocytosis, so the cells do not behave normally.
B. They resist lysis in even distilled water
No red cell resists lysis in distilled water.
C. They lyse at a higher saline concentration than normal cells (correct)
Correct. Spherocytes have little surface reserve, so they lyse in mildly hypotonic saline.
D. They lyse at a lower saline concentration than normal cells
Lysis at lower saline concentration (decreased fragility) is seen with target cells and thalassaemia.

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 Physiology question

What is the answer to this Physiology question?

The best answer is C, They lyse at a higher saline concentration than normal cells. Spherocytes have lost membrane surface area, for example from defects of spectrin or ankyrin, so they have little reserve to swell.

Why is "They behave exactly like normal cells in saline" not the answer?

The test is abnormal in hereditary spherocytosis, so the cells do not behave normally.

Why is "They resist lysis in even distilled water" not the answer?

No red cell resists lysis in distilled water.

Why is "They lyse at a lower saline concentration than normal cells" not the answer?

Lysis at lower saline concentration (decreased fragility) is seen with target cells and thalassaemia.

What concept does this question test?

It tests Blood and immunity: osmotic fragility in Physiology. They burst after taking in only a small amount of water, which means increased osmotic fragility. Target cells and thalassaemic cells show the opposite, decreased fragility.

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 Blood and immunity: osmotic fragility?

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.