Final answer:
A premature stop codon mutation can create a nonfunctional ADA enzyme, causing the build-up of deoxyadenosine.
Step-by-step explanation:
A mutation that creates a premature stop codon would result in a nonfunctional ADA enzyme. This is because the stop codon terminates the synthesis of the protein before it is fully formed, rendering it nonfunctional. The build-up of deoxyadenosine occurs because the nonfunctional ADA enzyme is unable to break it down.
For example, if a premature stop codon is introduced in the middle of the ADA gene during protein synthesis, the resulting protein will be truncated and unable to perform its function. Consequently, deoxyadenosine will accumulate and cause issues.