<prompt> Mechanism of action of sildenafil isInhibits PDE 2Inhibits PDE 4Inhibits PDE 5Inhibits PDE 3  <response>: Ans. is c i.e., Inhibits PDE 5 Sildenafil Viagra acts to increase cGMP by inhibiting its breakdown by phosphodiesterase isoform 5 PDE-5. 