Read the case, look at the photo, and determine the most likely cause of Rosie's insulin resistance.
Rosie, a 10-year-old 26.5-lb (12-kg) spayed female bichon frise, initially presented with marked polyuria, polydipsia, and polyphagia. Nonketotic diabetes mellitus was diagnosed based on the severe hyperglycemia and glucosuria, and NPH insulin was begun (0.5 U/kg, subcutaneously b.i.d.). In the next three months, Rosie's diabetes became more difficult to regulate; hyperglycemia persisted despite a progressive increase in insulin dosage (to 2.5 U/kg subcutaneously b.i.d.). Her polyuria had worsened, and the owner began noticing hair loss and weight gain. Physical examination revealed a body condition score of 7/9, hepatomegaly, and a mild pendulous abdomen. Noninflammatory hair thinning of the lateral trunk, alopecia of the ventral abdomen and tail, and multiple comedomes were noted. Routine laboratory tests revealed hyperglycemia, hypercholesterolemia, and high serum alkaline phosphatase activity. The serum concentrations of both total and free T4 were markedly low, but the TSH concentration was normal.
What is the most likely cause of Rosie's insulin resistance, obesity, and dermatologic clinical signs?