What is the size of the small obstruction that is excused for one afflicted with obsessive doubts (mawsūs), and is an obstruction the size of a mustard seed on the body considered small? What is the ruling on ritual purity (ṭahārah) with dangling skin flakes still attached to the skin, and is it sufficient to expect water to reach underneath them?

There is no obligation to remove attached dead skin flakes during Wudu', because they are considered part of the outer body. Some scholars have made concessions for minor impediments, such as what might be found under a fingernail or eye discharge. This is what Ibn Taymiyyah preferred, that a small amount of dirt under the fingernail or similar, if it prevents water from reaching, does not matter. As stated in "Al-Iqna'" and "Matalib Uli an-Nuha," a small amount ofakh under the fingernail or in the nose does not matter, even if it prevents water from reaching, because it is something that frequently occurs. Sheikh Taqi ad-Din also included every small impediment that prevents water from reaching, such as blood or dough on any part of the body, by analogy with what is under the fingernail.