Question 243

Question:

A 3-month-old male infant is brought to the physician because of recurrent viral infections and rashes over his trunk. Lymph nodes are difficult to detect on physical examination; imaging studies indicate the lack of a thymus. Urine deoxyadenosine concentration is 100 times greater than normal. A peripheral blood smear shows a marked decrease in both mature B and T lymphocytes. A deficiency of which of the following enzymes is most likely in this patient?

Correct Answer: B.  Adenosine deaminase

Severe combined (T and B) immunodeficiency, also known as SCID, is caused a purine salvage pathway deficiency of (B) adenosine deaminase. Without this enzyme to convert adenosine to inosine, there is an excess of ATP, dATP, and S-adenosylhomocysteine. These substances are toxic to immature lymphoid cells, and also prevent DNA synthesis through feedback inhibition of ribonucleotide reductase. Therefore, lymphoid cells fail either to reach maturity or to be produced in the first place, resulting in SCID.

Deficiency of (A) adenine phosphoriboxyltransferase (APRT) may lead to kidney stones formed of adenine and salts.

Deficiency of (C) adenosine kinase is associated with nonspecific developmental abnormalities.

(D) Adenylosuccinate synthetase deficiency has been implicated in the development of hyperuricemia and gout.

Deficiency of (E) Hypoxanthine-guanine phosphoribosyltransferase causes Lesch-Nyhan Syndrome, characterized by self-mutilation, retardation, aggression, hyperuricemia, and gout.