An 8-year-old boy presented with generalized darkening of skin of 1 year duration. His mother also complained of progressive weakness, weight loss, fatigue, poor appetite, nausea and occasional vomiting. There were two episodes of syncope also. There was no history of tuberculosis or any other systemic major illness in past. There was no history of any drug therapy preceding these complaints. He was hospitalized once for hypotension, shock and weakness. On examination, his blood pressure was 80/50 mmHg. All other vital parameters were within normal limits. On cutaneous examination, there was generalized hyperpigmentation of the skin, specially the face, hands, and palms (Fig. 14.1). The palmar creases and nail bed also showed hyperpigmentation and melanonychia was observed in a few nails (Fig. 14.2). Tongue and buccal mucosa showed patchy hyperpigmentation. Biochemical tests revealed hyponatremia (129 mmol/L) with hyperkalemia (5.5 mmol/L). 8 a.m. serum cortisol level was decreased to 46 nmol/L (range: 102–535 nmol/L). Liver and renal function tests, Thyroid stimulating hormone and blood glucose were within normal limits. Complete blood counts, RBC indices, peripheral blood smear and vitamin B12 level estimation did not reveal any abnormality. What is your diagnosis?