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Chapter 4: Nucleic Acids

You are treating a 4-year-old child who has been afflicted with recurrent infections. The child exhibits retarded development, weakness, and weight loss. Physical examination also shows absent tonsils and blood work reveals a lack of lymphocytes. These signs and symptoms are typical of severe combined immunodeficiency syndrome. Which of the following compounds would be expected to be elevated in the blood of this patient?

A. bilirubin

B. cytosine

C. deoxyadenosine

D. deoxythymidine

E. xanthine

Answer C: Severe combined immunodeficiency disease (SCID) is a disorder related to defects in the activity of the purine nucleotide catabolism enzyme, adenosine deaminase (ADA). In ADA deficiency there is an elevation in the level of adenosine and 2′-deoxyadenosine in the blood and 2′-deoxyadenosine levels in the urine are also elevated. The consequences of the elevations in these two ADA substrates are impaired lymphocyte differentiation, function, and viability, which results in lymphopenia and severe immunodeficiency. Increases in 2′-deoxyadenosine, through the action of ubiquitous nucleoside phosphorylases, results in dramatic increases in cellular dATP pools. The consequences of increased dATP pools are an inhibition of ribonucleotide reductase (RR), the enzyme responsible for generating deoxyribonucleotides (necessary for DNA replication) from ribonucleotides.

You are examining a 7-year-old male child who exhibits delayed motor development, overexaggerated reflexes, and spasticity. In addition, the boy has a tendency to chew his lips and fingers often causing tissue damage and bleeding. He also has painful, swollen, and inflamed joints. The most likely diagnosis in this case is Lesch-Nyhan syndrome. Which of the following compounds would be expected to be elevated in the urine of this patient?

A. adenosine

B. guanine

C. hypoxanthine

D. uric acid

E. xanthine

Answer D: Lesch-Nyhan syndrome results from deficiencies in hypoxanthine-guanine phosphoribosyltransferase (HGPRT). HGPRT is a purine nucleotide salvage enzyme that converts guanine to GMP and hypoxanthine to inosine 5'-monophosphate (IMP). Loss of this salvage enzyme results in increased rates of purine nucleotide catabolism whose end product is uric acid.

You are examining a 5-year-old male child who has been exhibiting a tendency to bite his lower lip to the point of bleeding as well as chewing on his fingertips. Physical examination demonstrates swollen tender joints. These symptoms and lab results are indicative of a deficiency in HGPRT. Given this enzyme deficiency, which of the following compounds would be expected to be elevated in the serum of this ...

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