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: Pharmacology
Mechanism of action of theophylline in bronchial asthma is 
Select and explain. 
A. Phosphodiestesterase 4 inhibition, B. inhibit the release of mediators, C. Anticholinergic action, D. Inhibition of mucociliary clearance
A: Phosphodiestesterase 4 inhibition
Answer: A: Phosphodiestesterase 4 inhibition 
Explanation: Mechanism of action of methylxanthines like theophylline, caffeine: (a) Release of Calcium from the sarcoplasmic reticulum, especially in skeletal and cardiac muscle. (b) Inhibition of phosphodiesterase3 and PDE4 which degrades cyclic nucleotides intracellularly. (c) Blockade of adenosine receptors: adenosine acts as a local mediator in CNS, CVS and other organs-contracts smooth muscles, especially bronchial; dilates cerebral blood vessels, depresses cardiac pacemaker and inhibits gastric secretion. Beta 2 agonism is also correct because Theophylline acts synergistic with beta 2 agonistsPharmacology and pharmacotherapeutics Satoskar 24th page no. 579 ESSENTIALS OF MEDICAL PHARMACOLOGY 7TH EDITION KD TRIPATHI Page No:225,226