What is the most appropriate management?

An 18-year-old man with cystic fibrosis was referred to clinic. Over recent months his lung function had deteriorated and he had lost weight. He was being treated for a chest infection at the time of the consultation.

Investigations:

oral glucose tolerance test (75 g):

fasting plasma glucose8.2 mmol/L (3.0C6.0)

2-h plasma glucose13.5 mmol/L (<7.8)

What is the most appropriate management?
A . repeat oral glucose tolerance test after chest infection has resolved
B . restrict refined carbohydrate intake
C . start gliclazide
D . start insulin
E . start sitagliptin