In which treatment method is functional $ADA$ (Adenosine Deaminase) injected into the patient via a syringe?

$E. coli$ is useful in the production of:

Statements related to human insulin are given below. Which statement$(s)$ is/are correct about genetically engineered insulin?
$(a)$ Pro-hormone insulin contains an extra stretch of $C$-peptide.
$(b)$ $A$-peptide and $B$-peptide chains of insulin were produced separately in $E. coli$,extracted,and combined by creating disulfide bonds between them.
$(c)$ Insulin used for treating diabetes was extracted from cattle and pigs.
$(d)$ Pro-hormone insulin needs to be processed for converting into a mature and functional hormone.
$(e)$ Some patients develop allergic reactions to the foreign insulin.
Choose the most appropriate answer from the options given below:

Which of the following is a main challenge for the production of insulin by the $r-DNA$ techniques?

Adenosine deaminase $(ADA)$ deficiency is most commonly caused by which genetic abnormality?

What does the conventional method of diagnosis include?