Should one make a new bracha of al netilas yadayim upon washing his hands after defecating within a meal?
One should not be quick to assume that one should make a new "al netilas yadayim". Although MB says in BH that one should, and Aruch Hashulhan says that no one would say that you should not say the bracha if you defecated, he must have missed the Taz (and the Maharshal) who brings a proof from the Rema that the opinion of the Maharshal to not make a new netilas yadayim within one seudah would apply to leaving to defecate as well. Because of the Taz's reasoning, SA HaRav holds one should not make the bracha based on the principle "safek brachos lehakel.
My mesora (heard from my father who heard it from Rav Ruderman ZT"L) is that in the Slobodka Yeshiva they did not pasken (decide halacha) from the Biur Halacha. It was learned for its lomdus (dialectics) but for a final decision they would go to the Kovna Rav.
The bottom line, (if you do not have a tradition in this matter) is:
Ask your Rav what he holds before you end up in such a situation and go with that.