Final answer:
A successful therapeutic effect of intranasal desmopressin (DDAVP) for a patient with diabetes insipidus would be indicated by a decrease in excessive thirst, as the medication promotes water reabsorption in the kidneys.
Step-by-step explanation:
When monitoring a patient with diabetes insipidus who is receiving intranasal desmopressin (DDAVP), a successful therapeutic effect would be indicated by decreased thirst. This is because desmopressin acts as a synthetic analogue of Antidiuretic Hormone (ADH), and its administration helps to reduce the excessive urination and thirst associated with the condition. Since ADH's primary function is to promote reabsorption of water in the kidneys, effective treatment with DDAVP should result in a decrease in the polyuria and polydipsia symptoms of diabetes insipidus.