What is the most important diagnosis to consider?

A 20-year-old man presented with a 6-month history of lethargy and weakness. His brother had been found to have adrenal failure at the age of 18. He had two sisters who were well and there was no other family history of endocrine autoimmune disease.

On examination, his blood pressure was 100/60 mmHg.

Investigations:

serum sodium136 mmol/L (137C144)

serum potassium4.8 mmol/L (3.5C4.9)

short tetracosactide (Synacthen®) test (250 micrograms):

baseline serum cortisol100 nmol/L

serum cortisol (30 min after tetracosactide)250 nmol/L (>550)

anti-adrenal antibodiesnegative

What is the most important diagnosis to consider?
A . adrenoleucodystrophy
B . autoimmune hypoadrenalism
C . familial glucocorticoid resistance
D . isolated adrenocorticotropic hormone deficiency
E . tuberculosis