This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
Which of the following drugs has been found to be useful in acute severe asthma ? 
Select and explain. 
A. Magnesium Sulphate, B. Anti-leukotrine, C. Cromolyn Sodium, D. Cyclosporine
A: Magnesium Sulphate
Answer: A: Magnesium Sulphate 
Explanation: A high concentration of oxygen should be given by face mask to achieve oxygen saturation of >90%.Magnesium sulfate given intravenously or by nebulizer has been found to be effective in acute severe asthma,when added to inhaled beta2 -agonists & is well tolerated.It can provide additional bronchodilation in patients whose presenting PEF(peak ejection fraction)is below 30% predicted. Reference:Harrison's medicine-18th edition,page no:2113;Davidson's medicine-22nd edition,page no:672.