Lymph nodes are difficult to detect on physical examination

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?

Adenine phosphoribosyltransferase
Adenosine deaminase
Adenosine kinase
Adenylosuccinate synthetase
Hypoxanthine-guanine phosphoribosyltransferase