General physiology: receptor-mediated endocytosis: 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 · General physiology: receptor-mediated endocytosis

Sample item

A 10-year-old boy has tendon xanthomas and a plasma LDL cholesterol of 600 mg/dL. His LDL receptors reach the cell surface and bind LDL normally, but fail to cluster in coated pits. Which process is defective?

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

The answer and the reasoning

C. Clathrin-mediated endocytosis of the receptor-LDL complex, which is an energy-requiring process

LDL is taken up by receptor-mediated endocytosis: receptor-ligand complexes cluster in clathrin-coated pits, which pinch off as vesicles in an ATP-dependent step. Failure of clustering leaves LDL in plasma, as in internalisation-defective LDL receptor mutations of familial hypercholesterolaemia. In the endosome the receptor is recycled and LDL is delivered to lysosomes.

Option by option

  • A. Sodium-coupled secondary active transport of cholesterol

    Cholesterol esters in LDL are not carried by sodium-coupled transporters.

  • B. Exocytosis of VLDL particles from hepatocytes

    VLDL secretion is an exit pathway and does not explain failure to clear LDL.

  • C. Clathrin-mediated endocytosis of the receptor-LDL complex, which is an energy-requiring process (correct)

    Correct. Without coated pit clustering the particle cannot be internalised.

  • D. Passive diffusion of LDL across the lipid bilayer down its gradient

    LDL is a large lipoprotein particle that cannot diffuse through the bilayer.

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, Clathrin-mediated endocytosis of the receptor-LDL complex, which is an energy-requiring process. LDL is taken up by receptor-mediated endocytosis: receptor-ligand complexes cluster in clathrin-coated pits, which pinch off as vesicles in an ATP-dependent step.

Why is "Sodium-coupled secondary active transport of cholesterol" not the answer?

Cholesterol esters in LDL are not carried by sodium-coupled transporters.

Why is "Exocytosis of VLDL particles from hepatocytes" not the answer?

VLDL secretion is an exit pathway and does not explain failure to clear LDL.

Why is "Passive diffusion of LDL across the lipid bilayer down its gradient" not the answer?

LDL is a large lipoprotein particle that cannot diffuse through the bilayer.

What concept does this question test?

It tests General physiology: receptor-mediated endocytosis in Physiology. Failure of clustering leaves LDL in plasma, as in internalisation-defective LDL receptor mutations of familial hypercholesterolaemia. In the endosome the receptor is recycled and LDL is delivered to lysosomes.

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 General physiology: receptor-mediated endocytosis?

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.