What is the most appropriate next investigation?

A 17-year-old girl presented with primary amenorrhoea. She had grown and developed normally. There was no history of galactorrhoea or hirsutism.

On examination, her height was 1.69 m, her weight was 68.3 kg, and her body mass index was 23.9 kg/m2 (18C25). She had stage 5 breast development and stage 5 pubic hair. Her visual fields were full to confrontation.

Investigations:

serum cortisol (09.00 h)416 nmol/L (200C700)

serum oestradiol51 pmol/L (200C400)

serum follicle-stimulating hormone0.8 U/L (2.5C10.0)

serum luteinising hormone1.2 U/L (2.5C10.0)

serum thyroid-stimulating hormone1.2 mU/L (0.4C5.0)

serum free T415.6 pmol/L (10.0C22.0)

What is the most appropriate next investigation?
A . karyotyping
B . luteinising hormone-releasing hormone test
C . serum insulin-like growth factor 1
D . serum prolactin
E . short tetracosactide (Synacthen®) test