The leaves of a tree/plant consists of chloroplast. Hence, if summer turns fall it is the leaves that would lose most chloroplast. Chloroplast consists of chlorophyll that is useful in the process of photosynthesis.
Not only this during fall season the leaves even fall from the branches of the tree. Hence, for any green plant loss of chloroplast occurs mainly in the leaves and in a large amount
An independent variable is the only variable that is changed while the rest is kept the same. In this setup, the doctor is using different creams on the same patient with the same skin condition so the independent variable is the skin cream and not the skin condition.