Pathophysiology question

[cleo_md]

A patient with small cell carcinoma of the lung complains of muscle weakness , fatigue , confusion and weight gain . Physical examination is unremarkable . Serum sodium is found to be 120 mEq/L . Which of the following abnormal laboratory results would also be expected in this patient .

a-decreased plasma atrial natriutric peptide ( ANP) concentration
b-decreased plasma vasopressin concentration
c-decreased serum osmolarity
d-decreased urinary sodium concentration
e-increased plasma aldosterone concentration

[dr_b.e.s.m]

I think this is SIADH and the correct answer is C? [decreased serum osmolarity].

the small cell carcinoma produces vasopressin (para malignant syndrome) leading to water retention , hyponatremia , dec. serum osmolarity , inc. Na excretion in urine , inc. urine osmolarity. renin and aldosterone level will be low as there is hypovolemia. I think ANP also will be low due to the hyponatremia.