Immunity: antigen processing: 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 · Immunity: antigen processing

Sample item

A child has a mutation of the transporter associated with antigen processing (TAP), with low MHC class I surface expression and recurrent respiratory infections. Which step is impaired?

ACross-linking of surface immunoglobulin on B cells by antigenBBinding of CD4 to the nonpolymorphic region of MHC class IICDelivery of cytosolic peptides into the endoplasmic reticulum for loading onto MHC class IDRemoval of the invariant chain from MHC class II in the endosome

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

The answer and the reasoning

C. Delivery of cytosolic peptides into the endoplasmic reticulum for loading onto MHC class I

Endogenous proteins are degraded by the proteasome and the peptides are pumped into the ER by TAP, where they bind MHC class I with the help of tapasin and calreticulin. The complex presents antigen to CD8 cytotoxic T cells. Failure produces bare lymphocyte syndrome type I.

Option by option

A. Cross-linking of surface immunoglobulin on B cells by antigen
B-cell receptor cross-linking is part of B-cell activation and is not part of the TAP pathway.
B. Binding of CD4 to the nonpolymorphic region of MHC class II
CD4 binding to MHC class II belongs to helper T-cell recognition, not the MHC class I pathway.
C. Delivery of cytosolic peptides into the endoplasmic reticulum for loading onto MHC class I (correct)
Correct. TAP carries proteasome-derived peptides into the ER for MHC class I loading.
D. Removal of the invariant chain from MHC class II in the endosome
Invariant chain handling concerns MHC class II, which presents exogenous antigen to CD4 cells.

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, Delivery of cytosolic peptides into the endoplasmic reticulum for loading onto MHC class I. Endogenous proteins are degraded by the proteasome and the peptides are pumped into the ER by TAP, where they bind MHC class I with the help of tapasin and calreticulin.

Why is "Cross-linking of surface immunoglobulin on B cells by antigen" not the answer?

B-cell receptor cross-linking is part of B-cell activation and is not part of the TAP pathway.

Why is "Binding of CD4 to the nonpolymorphic region of MHC class II" not the answer?

CD4 binding to MHC class II belongs to helper T-cell recognition, not the MHC class I pathway.

Why is "Removal of the invariant chain from MHC class II in the endosome" not the answer?

Invariant chain handling concerns MHC class II, which presents exogenous antigen to CD4 cells.

What concept does this question test?

It tests Immunity: antigen processing in Physiology. The complex presents antigen to CD8 cytotoxic T cells. Failure produces bare lymphocyte syndrome type I.

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 Immunity: antigen processing?

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.