Under normal downward sloping demand curve and fully elastic supply curve of a commodity, an exogenous decrease in demand would lead to

When the supply curve is perfectly (fully) elastic, it is horizontal at a fixed price. Any shift in the demand curve only changes the equilibrium quantity, not the equilibrium price. An exogenous decrease in demand shifts the demand curve leftward, intersecting the horizontal supply curve at a lower quantity but at the same price. Hence equilibrium quantity decreases and price remains unchanged.