An 18-month old male baby has been having reoccurring infections for the last year and is now starting to show neurological complications. Genetic testing shows a defect for a metabolic enzyme. The child’s ammonia and uric acid levels are normal. Blood pH is 7.41. The kidney function is also normal. A defect in which of the following enzymes is most likely to show the above symptoms?
a. Purine nucleoside phosphorylase
b. Xanthine oxidase
c. D-amino acid oxidase
d. Arginase
e. Phenylalanine hydroxylase

There is a defect in Purine nucleoside phosphorylase enzyme in that child.

Explanation:  

This enzyme helps in Purine metabolism. It converts inosine to hypoxanthine and guanosine to guanine.   The deficiency of this enzyme causes immunodeficiency disorder where the immune system is unable to protect our body properly from foreign pathogens such as bacteria and viruses. Some people have a low amount of T cells because of the deficiency of this enzyme. T cell recognizes the pathogens and attacks them to prevent infection. Some people have a low amount of B cells, which helps us to fight against the infections by producing antibodies or immunoglobulins. Antibodies target foreign pathogens and destroy them. Some very affected individuals lack both T cells and B cells. They go through have a serious condition known as severe combined immunodeficiency (SCID).