Skeletal dysplasias: pycnodysostosis: 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.

Try it

Orthopaedics · Skeletal dysplasias: pycnodysostosis

Sample item

A short-statured child has dense bones that fracture easily, persistently open anterior fontanelle, an obtuse mandibular angle and terminal phalangeal acro-osteolysis. Which enzyme is deficient?

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

The answer and the reasoning

B. Cathepsin K

Pycnodysostosis is an autosomal recessive sclerosing dysplasia caused by cathepsin K deficiency, which impairs osteoclast bone resorption. Features are short stature, brittle sclerotic bones, open fontanelles, obtuse mandibular angle and acro-osteolysis. It is distinguished from osteopetrosis by normal marrow function.

Option by option

  • A. Alkaline phosphatase

    Alkaline phosphatase deficiency causes hypophosphatasia with poor mineralisation.

  • B. Cathepsin K (correct)

    Correct. Cathepsin K deficiency causes pycnodysostosis.

  • C. Alpha-L-iduronidase

    Alpha-L-iduronidase deficiency causes Hurler syndrome.

  • D. Carbonic anhydrase II

    Carbonic anhydrase II deficiency causes a form of osteopetrosis with renal tubular acidosis.

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.

More Orthopaedics questions

Questions about this Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is B, Cathepsin K. Pycnodysostosis is an autosomal recessive sclerosing dysplasia caused by cathepsin K deficiency, which impairs osteoclast bone resorption.

Why is "Alkaline phosphatase" not the answer?

Alkaline phosphatase deficiency causes hypophosphatasia with poor mineralisation.

Why is "Alpha-L-iduronidase" not the answer?

Alpha-L-iduronidase deficiency causes Hurler syndrome.

Why is "Carbonic anhydrase II" not the answer?

Carbonic anhydrase II deficiency causes a form of osteopetrosis with renal tubular acidosis.

What concept does this question test?

It tests Skeletal dysplasias: pycnodysostosis in Orthopaedics. Features are short stature, brittle sclerotic bones, open fontanelles, obtuse mandibular angle and acro-osteolysis. It is distinguished from osteopetrosis by normal marrow function.

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 Skeletal dysplasias: pycnodysostosis?

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.