i) Based on the information provided, the most likely diagnosis for the patient is Cushing's syndrome. This is indicated by the symptoms of weight gain, facial hirsutism, thinning of hair, spontaneous bruising, and muscle weakness. The elevated fasting plasma glucose and hypertension also suggest an endocrine disorder. The underlying cause of Cushing's syndrome in this case is likely to be an excess production of cortisol. This can be caused by either an adrenal adenoma (a tumor in the adrenal gland) or a pituitary adenoma (a tumor in the pituitary gland). Further tests are needed to determine the exact cause. ii) The significance of 9am cortisol measurements is that cortisol levels are typically highest in the morning and lowest at night. By measuring cortisol levels in the morning, it helps to assess the body's natural cortisol production and its diurnal rhythm. In Cushing's syndrome, cortisol levels are usually elevated throughout the day, including in the morning. iii) The results seem to exclude the possibility of an adrenal adenoma because the plasma ACTH level is elevated (504 ng/l) and the serum cortisol level is also elevated (752 nmol/l) at 9am. In cases of adrenal adenoma, the ACTH level is typically low or undetectable, and the cortisol level is usually high. The elevated ACTH level suggests a pituitary cause for the excess cortisol production. iv) Additional tests that might be performed to confirm the diagnosis and distinguish pituitary from extra-pituitary causes include: - High-dose dexamethasone suppression test: This test involves administering a high dose of dexamethasone, a synthetic glucocorticoid, and measuring cortisol levels before and after. In Cushing's syndrome, cortisol levels will not be suppressed by the dexamethasone in pituitary causes, but will be suppressed in extra-pituitary causes. - CRH stimulation test: This test involves administering corticotropin-releasing hormone (CRH) and measuring ACTH and cortisol levels before and after. In pituitary causes, there will be an exaggerated response of ACTH and cortisol levels to CRH stimulation, while in extra-pituitary causes, the response will be blunted. - Imaging studies: Imaging studies such as MRI or CT scans may be performed to visualize the pituitary gland and adrenal glands, looking for any tumors or abnormalities. - Inferior petrosal sinus sampling: This is an invasive procedure where blood samples are taken from the veins draining the pituitary gland and the adrenal glands to measure ACTH levels. It helps to differentiate between pituitary and ectopic ACTH production. These additional tests will help to confirm the diagnosis of Cushing's syndrome and determine the underlying cause.

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