NDA (II) 2017 General Ability Test · Question 66 of 150

Correct — B, Nucleic acids. Gout is a disorder of purine metabolism. Uric acid is the final product when the body breaks purines down, and adenine and guanine, two of the bases of DNA and RNA, are purines. Dietary nucleic acids are therefore digested to purines and then to uric acid, and when the level in the blood rises far enough the acid crystallises as monosodium urate in and around a joint, producing the sudden, intensely painful swelling of gouty arthritis, classically at the base of the big toe. This is why the standard dietary advice is to cut down on purine-rich foods — organ meat such as liver and kidney, red meat, small oily fish and shellfish, and beer, all of which are heavy in nucleic acid or in what its breakdown produces.

• (a)Food fibres — Dietary fibre is largely indigestible plant carbohydrate. It yields no purines at all and there is no reason to restrict it; if anything a gout patient is encouraged to eat more vegetables and whole grains.
• (c)Lipids — Fats are not a source of purines, so they do not raise uric acid directly. Weight control matters in gout, but the food component that has to be cut for this particular reason is nucleic acid, not fat.
• (d)Carbohydrates — Ordinary carbohydrates contain no purines. Sugary drinks sweetened with fructose are a recognised risk factor by a separate route, but the question asks what has to be minimised because of the uric acid pathway, and that is the nucleic acid.

Purines and pyrimidines are the two families of nitrogen bases in nucleic acids. Adenine and guanine are purines; cytosine, thymine and uracil are pyrimidines. Humans lack the enzyme uricase that many other mammals use to break uric acid down further, so uric acid is our end point for purines and is excreted mainly by the kidney. Trouble arises either when too much is produced or when too little is cleared, and the crystals it forms are what the joint reacts to.

Work backwards from the uric acid named in the stem. Uric acid comes from purines, purines come from nucleic acids, and only one option on the list is a nucleic acid. Nothing else offered feeds that pathway. It is worth knowing that diet is only part of the story — most of the body's uric acid comes from its own cell turnover rather than from food, and drugs that block the enzyme xanthine oxidase are the mainstay of long-term treatment. Note also that hyperuricaemia and gout are not the same thing: many people have a raised level without ever having an attack.

• Uric acid is the final product of purine metabolism in humans.
• Adenine and guanine, the purine bases, are components of DNA and RNA.
• Gout occurs when uric acid crystallises as monosodium urate in and around joints.
• Purine-rich foods include organ meat, red meat, small oily fish, shellfish and beer.
• Humans lack uricase, the enzyme with which many other mammals degrade uric acid further.

• Cutting fats or carbohydrates because they sound unhealthy; neither yields purines.
• Assuming diet is the whole cause of gout, when most uric acid comes from the body's own cell turnover.
• Treating a raised uric acid level as identical to gout; the disease needs crystals in a joint.

NDA links a named disease to the nutrient behind it, so learn the metabolic chain — food component, intermediate, waste product — rather than a bare list of foods.

• practice — not a real PYQ

  Uric acid in the human body is the end product of the metabolism of

  • (a)Purines
  • (b)Fatty acids
  • (c)Pyrimidines
  • (d)Glucose

  Answer(a) Purines — adenine and guanine are broken down to uric acid, which humans cannot degrade further.

• practice — not a real PYQ

  Which of the following is a purine base found in DNA?

  • (a)Guanine
  • (b)Cytosine
  • (c)Thymine
  • (d)Uracil

  Answer(a) Guanine — adenine and guanine are the purines, while cytosine, thymine and uracil are pyrimidines.