What is the most likely diagnosis for a patient with polyuria, nocturia, and normal serum sodium but low urinary osmolality?

The presence of polyuria, nocturia, and low urinary osmolality alongside normal serum sodium is indicative of Diabetes insipidus. In this condition, the kidneys fail to concentrate urine due to insufficient levels of antidiuretic hormone (ADH) or a lack of response to it. This leads to the excretion of large volumes of dilute urine, resulting in symptoms such as excessive urination and increased nighttime urination (nocturia).

In patients with Diabetes insipidus, despite the high urinary output, the serum sodium remains normal because the kidneys are still able to excrete sodium in the presence of the dilute urine. The primary focus on low urinary osmolality reflects the inability of the kidneys to concentrate urine effectively.

This clinical picture helps differentiate Diabetes insipidus from other conditions that may also present with polyuria. For example, compulsive polydipsia may mimic some symptoms, but it usually leads to lower serum sodium levels due to excess water intake. Diabetes mellitus, while associated with polyuria, typically results in high urinary osmolality due to the presence of glucose in the urine. The syndrome of inappropriate antidiuretic hormone (SIADH) would generally lead to elevated urinary osmolality and low serum sodium