A 28-year-old woman presents to the ENT clinic with a 3-year history of nasal congestion, rhinorrhoea, and sneezing that worsens during the monsoon season. She reports itching of the nose and palate. Examination reveals pale, boggy nasal mucosa with clear nasal discharge. Nasal smear shows predominance of eosinophils (>5%). She has tried intranasal saline rinses with minimal relief. What is the most appropriate first-line pharmacological treatment for this patient?