Final answer:
The best indication of adequate treatment with phenoxybenzamine is the stabilization of blood pressure, not side effects like nasal stuffiness or hypovolaemia.
Step-by-step explanation:
The best indication of a patient that is adequately treated with phenoxybenzamine is C. Stabilisation of blood pressure. Phenoxybenzamine is a nonselective alpha-adrenergic antagonist used to treat conditions like pheochromocytoma, where excessive catecholamines can lead to hypertension. Effective treatment would therefore result in normalized or stabilized blood pressure levels. Options such as 'complaint of nasal stuffiness' or 'postural hypotension' may suggest excessive or insufficient pharmacologic effects leading to side effects or inadequate blood pressure control. Similarly, 'decreased haematocrit' and 'hypovolaemia' are not direct indicators of effective treatment with phenoxybenzamine.