Bone tumours: CD99 in small round cell tumours: practice question

A Orthopaedics 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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Orthopaedics · Bone tumours: CD99 in small round cell tumours

Sample item

A 13-year-old has a painful diaphyseal femoral lesion. Biopsy shows sheets of uniform small round blue cells with PAS-positive, diastase-sensitive cytoplasm. Which immunohistochemical marker is expected to show strong, diffuse membrane staining?

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

The answer and the reasoning

D. CD99

Ewing sarcoma shows strong membranous CD99 (MIC2) staining and glycogen-rich cytoplasm. Desmin marks rhabdomyosarcoma, CD45 marks lymphoma, and synaptophysin marks neuroblastoma, which are the other small round blue cell tumours of childhood.

Option by option

  • A. CD45

    CD45 is the leucocyte common antigen and marks lymphoma.

  • B. Synaptophysin

    Synaptophysin is a neuroendocrine marker positive in neuroblastoma.

  • C. Desmin

    Desmin is positive in rhabdomyosarcoma.

  • D. CD99 (correct)

    Correct. Diffuse membranous CD99 is characteristic of Ewing sarcoma.

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 Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is D, CD99. Ewing sarcoma shows strong membranous CD99 (MIC2) staining and glycogen-rich cytoplasm.

Why is "CD45" not the answer?

CD45 is the leucocyte common antigen and marks lymphoma.

Why is "Synaptophysin" not the answer?

Synaptophysin is a neuroendocrine marker positive in neuroblastoma.

Why is "Desmin" not the answer?

Desmin is positive in rhabdomyosarcoma.

What concept does this question test?

It tests Bone tumours: CD99 in small round cell tumours in Orthopaedics. Desmin marks rhabdomyosarcoma, CD45 marks lymphoma, and synaptophysin marks neuroblastoma, which are the other small round blue cell tumours of childhood.

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 Bone tumours: CD99 in small round cell tumours?

Pair each fracture with its classic complication and nerve injured. Then use the flashcards and the other Orthopaedics 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.