I'm in middle of davening, and scratch a usually-covered place using my little finger (pinky) on my right hand. This requires me to wash my hand before continuing. Do I have to:
- wash both my hands?
- wash my right hand?
- wash my pinky finger?
Let us first address a clearly delineated case: There is a law that when urinating in the middle of a meal and using only one hand to clean oneself, one need only wash that hand - Yoma 30a, Shul'han Arukh Ora'h 'Haim 170:1. No differentiation was made regarding the fingers used, so I would assume that in a meal it would be required to still wash the whole hand. So it is clear that you don't need to wash both hands.
Now regarding prayer: Nevertheless for praying as opposed to eating, since the requirement to wash one's hands fully besides once in the morning in order to pray (according to the opinion accepted as halacha) is not restrictive, and only their cleanliness is (ibid. 92:6), despite that a-priori halacha requires washing the whole hand (ibid. 92:5), nevertheless once already in the middle of prayer that one can't move in, an act of cleaning alone is enough. Therefore when in the middle of the "Amidah" it would seem that it would be enough to clean the finger (since we are only concerned with cleanliness and cleaning a clean part of the hand does nothing, as it is already clean). When in a part of prayer that permits walking around, one would have to wash the whole hand (Mishna Berura 92:27) since a priori, more than just "cleaning" is required - "washing". Of course one should not touch the dirty area of the body when in prayer (ram"a 92:7).
The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909), in his work of responsa "Torah Lishma," (Siman 23) clarifies that if one extends only one hand in the restroom, then he is required to wash only that hand; he does not have to wash both hands, since only one hand was exposed to the impurity of the restroom. Nevertheless, Chacham Bentzion Abba Shaul (Jerusalem, 1924-1998) writes his work "Or Le'tzion" (Vol. 2, 1:13) that even in such a case one should preferably wash both hands. He apparently understood that the impurity that descends upon the individual's hand spreads even to the second hand, and therefore one should wash both hands, even if only one hand had been in the restroom. He adds that even if a person placed just one finger in the restroom, he should wash both hands.