If a Patent claims more than one "Item", say Item A, Item B, Item C etc... and if it can be proved that say Item A, has been offered and sold to a commercial entity, can the Patent be invalidated in its totality, or only Item A is invalidated?
Ron's answer is basically correct. The part of a patent that might or might not be infringed is a claim. It is also a claim that might or might not be found invalid. As an example, a patent could have 20 or more claims, most of which are narrowed versions of a small handful of key, independent claims. One of the reasons for having a claim set with increasingly narrow claims is that the broader the claim the more susceptible to being found invalid due to something coming to light after the patent was granted. From the inventors point of view, the narrow claims stake out a fall-back position.
The only quibble with Ron's answer is that both independent and dependent claims can be invalidated, possibly leaving other independent and/or dependent claims still enforceable.