The patient is exhibiting signs of anaphylaxis, characterized by facial swelling, stridor, and hypotension following a bee sting. The immediate treatment is intramuscular (IM) adrenaline (epinephrine) 0.5 mg because it rapidly reverses airway obstruction, vasodilation, and hypotension by vasoconstriction, bronchodilation, and increasing cardiac output. Adrenaline is the first-line treatment and should be administered without delay to prevent progression to respiratory failure or cardiovascular collapse. Other therapies like hydrocortisone or antihistamines are adjuncts but have a delayed onset and are not substitutes for adrenaline. Nebulized salbutamol may help bronchospasm but does not address systemic vasodilation or shock.

Reference: Harrison’s Principles of Internal Medicine, Jameson et al., 20th Edition.