Pictures of the whole plant would still help tremendously. It does look like potassium def, but I don't believe it. Even looking at those pictures, the inside of the leaves are still quite green in those pictures. Where as yours looks to be more fading away completely to yellow. The middle finger on your leaf looks nearly completely yellow (you have full on dead yellow leaves in the back ground), It looks like the plant is hungry but the other leaves in the picture are nice green. As why I asked for a full picture.
The yellowing of the leaves like that lead me to think it's either hungry or it's just the natural process, plants eat the lower leaves for food for many reasons.