Essential fatty acids: deficiency: practice question

A Biochemistry 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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Biochemistry · Essential fatty acids: deficiency

Sample item

A 6-month-old infant on prolonged fat-free parenteral nutrition develops scaly dermatitis, hair loss, poor wound healing and thrombocytopenia. Deficiency of which nutrient is most likely, and why can the body not make it?

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

The answer and the reasoning

D. Linoleic acid, because humans lack desaturases that insert double bonds beyond carbon 9

Humans cannot insert double bonds distal to carbon 9 (no delta-12 or delta-15 desaturase), so linoleic (omega-6) and alpha-linolenic (omega-3) acids must come from the diet. Deficiency causes scaly skin, impaired growth and wound healing, and thrombocytopenia. Arachidonic acid, a precursor of eicosanoids, is made from linoleic acid.

Option by option

  • A. Palmitic acid, because humans lack fatty acid synthase

    Palmitic acid is made by fatty acid synthase, which is present in liver and adipose tissue.

  • B. Cholesterol, because humans lack HMG-CoA reductase

    Humans synthesise cholesterol using HMG-CoA reductase.

  • C. Oleic acid, because humans lack the delta-9 desaturase

    Oleic acid is synthesised by delta-9 desaturase, which humans possess.

  • D. Linoleic acid, because humans lack desaturases that insert double bonds beyond carbon 9 (correct)

    Linoleic acid is an essential fatty acid, and its absence explains the skin and platelet findings.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Linoleic acid, because humans lack desaturases that insert double bonds beyond carbon 9. Humans cannot insert double bonds distal to carbon 9 (no delta-12 or delta-15 desaturase), so linoleic (omega-6) and alpha-linolenic (omega-3) acids must come from the diet.

Why is "Palmitic acid, because humans lack fatty acid synthase" not the answer?

Palmitic acid is made by fatty acid synthase, which is present in liver and adipose tissue.

Why is "Cholesterol, because humans lack HMG-CoA reductase" not the answer?

Humans synthesise cholesterol using HMG-CoA reductase.

Why is "Oleic acid, because humans lack the delta-9 desaturase" not the answer?

Oleic acid is synthesised by delta-9 desaturase, which humans possess.

What concept does this question test?

It tests Essential fatty acids: deficiency in Biochemistry. Deficiency causes scaly skin, impaired growth and wound healing, and thrombocytopenia. Arachidonic acid, a precursor of eicosanoids, is made from linoleic acid.

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 Essential fatty acids: deficiency?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.