Glyceryl trinitrate (organic nitrate)

Well-done! Sildenafil is a phosphodiesterase inhibitor which potentiates the nitric oxide pathway. GTN being a nitric oxide donor, can interact with sildenafil via potentiation of the same pathway. Activation of the nitric oxide pathway leads to venodilation (reduced preload) and to a lesser extend vasodilation, resulting in low blood pressure.