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Option D

I'd go with option D.

In case of SIADH there will be high level of plasma ADH. This hormone increases absorption of water ( not solutes ) mainly from collecting duct.

As the tubular filtrate goes through proximal tubule, the absorption here will be isoosmotic i.e., concentration of the filtrate won't change.

Reaching ascending lope of Henle, there will be a high amount of absorption of solutes i.e., sodium, potassium, chloride, calcium...etc but not water because this segment is inpermeable to free water. This activity will make the filtrate dilute.

In distal tubule, there will be absorption sodium which will be accompanied by free water....

Finally in collecting tubule, the ADH works to absorb free water only... This final action will make the filtrate further concentrated.... To clarify the fact of selecting medullary collecting ducts, I think ( although am not certainly sure ) there is a system called counter current exchange system and this will make the filtrate even further concentrated....
 
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