NDA (II) 2017 General Ability Test · Question 103 of 150

Correct — D, Vitamin K. Clotting runs on a chain of plasma proteins, and several of them cannot work until vitamin K has modified them. Vitamin K acts as the cofactor for an enzyme that adds a carboxylic acid group to particular glutamate residues in prothrombin (factor II) and in factors VII, IX and X, along with proteins C, S and Z. Those modified residues are what let the proteins grip calcium ions, and without that grip they cannot assemble on a membrane surface and the cascade stalls, so bleeding goes uncontrolled. The letter K itself comes from Koagulation, the German for clotting.

• (a)Vitamin A — Vitamin A serves vision and the health of skin and mucous membranes; its deficiency gives night blindness and xerophthalmia, not bleeding.
• (b)Vitamin D — Vitamin D governs calcium and phosphate absorption and bone mineralisation, so its deficiency shows up as rickets in children and osteomalacia in adults. It supplies calcium to the body but plays no part in modifying the clotting proteins.
• (c)Vitamin E — Vitamin E is a fat-soluble antioxidant that protects cell membranes from oxidative damage. It is often confused with K because both are fat-soluble, but it has no clotting role.

The four fat-soluble vitamins are A, D, E and K, and each has a job that is easy to keep separate once you attach it to a body system: A to vision, D to bone and calcium, E to protecting membranes, K to blood clotting. Vitamin K's contribution is chemical and precise. It is the cofactor for gamma-carboxylation, a reaction that converts glutamate residues in certain proteins into gamma-carboxyglutamate, and only in that form can those proteins bind calcium and take part in coagulation.

Newborn babies illustrate the point better than any diagram. Their clotting factors run at only about 30 to 60 per cent of adult levels, vitamin K crosses the placenta poorly and breast milk carries very little of it, so an injection of vitamin K1 at birth is standard practice to prevent vitamin K deficiency bleeding. In adults a further supply comes from bacteria in the large intestine, which is why frank deficiency is uncommon except after prolonged antibiotic use or where fat absorption is impaired. The same chemistry explains a common medicine: warfarin works by blocking the recycling of vitamin K, thinning the blood on purpose.

• Vitamin K is the cofactor for gamma-carboxylation of prothrombin (factor II) and factors VII, IX and X, plus proteins C, S and Z.
• The carboxylated residues let these proteins chelate calcium ions, which they cannot do otherwise; without that step coagulation is seriously impaired.
• Newborns are given a vitamin K1 injection because their clotting factors are only about 30 to 60 per cent of adult values and both placental transfer and breast-milk content are low.
• Vitamin K1 comes mainly from green leafy vegetables; vitamin K2 is supplied partly by bacteria of the large intestine.
• A, D, E and K are the fat-soluble vitamins; the B group and vitamin C are water-soluble and are not stored in quantity.

• Pairing vitamin D with clotting because it deals with calcium; the calcium in coagulation is bound by K-modified proteins.
• Confusing E and K, which are both fat-soluble but do entirely different jobs.
• Assuming a healthy adult can never be short of vitamin K — long antibiotic courses or poor fat absorption can produce a deficiency.

NDA sets a one-line vitamin-to-function or vitamin-to-deficiency item almost every year, sometimes as a matching list.

• practice — not a real PYQ

  Deficiency of which one of the following vitamins causes rickets in children ?

  • (a)Vitamin A
  • (b)Vitamin C
  • (c)Vitamin D
  • (d)Vitamin K

  Answer(c) Vitamin D — poor calcium and phosphate absorption leaves the growing bones soft and deformed.

• practice — not a real PYQ

  Newborn infants are routinely given an injection of vitamin K mainly to prevent

  • (a)night blindness
  • (b)bleeding due to low clotting factors
  • (c)anaemia
  • (d)soft bones

  Answer(b) bleeding due to low clotting factors — a newborn's clotting factors are only about a third to a half of adult levels and both placental transfer and breast-milk content of vitamin K are low.