THEME AND FOCUS: METABOLISM OF NUCLEOTIDES Case Study: A 17- month- old girl suffered from recurring respiratory infections. Injection of Diphtheria-Peussis-Tetanus (DPT) and Typhoid vaccine produced only a minimal response. The lysate of girl's erythrocytes were found to lack detectable adenosine deaminase activity. Her mother and father both showed approximately 50% of the normal red cell adenosine deaminase activity. Lead Question: Diagnose the disease.