Patient has low serum sodium = hyponatremia.Given that the patient has a LOW URINE OSMOLARITY, it suggests that ADH is NOT active. The only way for someone to have hyponatremia AND a low ADH (in this case) is through psychogenic polydipsia (e.g. if it was SIADH, the urine would be MAXIMALLY concentrated and it is NOT in this case)
(A) would cause central DI -- no ADH means one develops hypernatremia as free water is lost in the urine, thus concentrating the serum.
(B) osmotic diuresis could cause hypernatremia due to loss of free water in the urine
(C) degradation of ADH leads to DI which means one develops hypernatremia
(E) resistance to ADH (nephrogenic DI), again, hypernatremia.
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