Gastrointestinal physiology: villus countercurrent exchange: 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 · Gastrointestinal physiology: villus countercurrent exchange

Sample item

A 70-year-old man with atrial fibrillation has acute abdominal pain from low mesenteric blood flow. Mucosal necrosis starts at the villus tips. What explains this early vulnerability?

AThe tips receive blood only through the portal veinBTip enterocytes have no mucus protectionCOxygen diffuses from the villus arteriole to the adjacent venule, so tip oxygen tension is low even at normal flowDVillus tips have no capillary supply

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

The answer and the reasoning

C. Oxygen diffuses from the villus arteriole to the adjacent venule, so tip oxygen tension is low even at normal flow

Within the villus, the central arteriole and the venules run in opposite directions in close contact, allowing oxygen to shunt back to the venous side. The tip oxygen tension is therefore low even at rest and falls critically when flow decreases. This makes villus tips the first part of the mucosa to be damaged in ischaemia.

Option by option

A. The tips receive blood only through the portal vein
Villus blood is arterial in origin; the portal vein only drains venous blood.
B. Tip enterocytes have no mucus protection
Mucus protection is not the reason for early ischaemic injury at the tip.
C. Oxygen diffuses from the villus arteriole to the adjacent venule, so tip oxygen tension is low even at normal flow (correct)
Correct. Countercurrent exchange leaves the tip poorly oxygenated.
D. Villus tips have no capillary supply
The tip is supplied by the capillary network that arises from the arteriole.

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, Oxygen diffuses from the villus arteriole to the adjacent venule, so tip oxygen tension is low even at normal flow. Within the villus, the central arteriole and the venules run in opposite directions in close contact, allowing oxygen to shunt back to the venous side.

Why is "The tips receive blood only through the portal vein" not the answer?

Villus blood is arterial in origin; the portal vein only drains venous blood.

Why is "Tip enterocytes have no mucus protection" not the answer?

Mucus protection is not the reason for early ischaemic injury at the tip.

Why is "Villus tips have no capillary supply" not the answer?

The tip is supplied by the capillary network that arises from the arteriole.

What concept does this question test?

It tests Gastrointestinal physiology: villus countercurrent exchange in Physiology. The tip oxygen tension is therefore low even at rest and falls critically when flow decreases. This makes villus tips the first part of the mucosa to be damaged in ischaemia.

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 Gastrointestinal physiology: villus countercurrent exchange?

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.