NEET 2021 Zoology Question 192 Solution

Adenosine Deaminase (ADA) is a housekeeping enzyme in the purine salvage pathway that catalyzes the irreversible deamination of adenosine and deoxyadenosine into inosine and deoxyinosine: - In the absence of functional ADA (due to autosomal recessive loss-of-function mutations/deletions on chromosome 20), its substrate deoxyadenosine accumulates intracellularly in lymphoid tissues. - Deoxyadenosine is phosphorylated into cytotoxic levels of deoxyadenosine triphosphate (dATP), which allosterically inhibits ribonucleotide reductase, stalling DNA replication and repair. - Immature thymocytes, mature T lymphocytes, and B lymphocytes undergo accelerated apoptosis, completely crippling both cell-mediated and humoral immunity (Severe Combined Immunodeficiency, SCID).

- Parkinson’s disease: A neurodegenerative motor disorder caused by loss of dopaminergic neurons in the substantia nigra of the basal ganglia. - Digestive disorders: Involve gastric, hepatic, or pancreatic enzyme deficiencies (e.g. celiac disease, lactase deficiency). - Addison’s disease: An endocrine disorder caused by autoimmune destruction of the adrenal cortex, leading to hyposecretion of glucocorticoids and mineralocorticoids.