Select the best definition of Anaphylaxis.

A. A rapid severe systemic hypersensitivity reaction that may compromise airway, breathing or circulation. B. Glomerular inflammation causing haematuria, reduced filtration and often hypertension. C. Acute infection of the meninges requiring urgent antimicrobial treatment. D. Heavy proteinuria with hypoalbuminaemia, oedema and often hyperlipidaemia.