Which condition presents with polydipsia, polyuria, and an inability to concentrate urine in the presence of elevated serum osmolality?

• A. Central diabetes insipidus
• B. Nephrogenic diabetes insipidus
• C. SIADH
• D. Hyperaldosteronism

Answer: (B)

The presence of polydipsia, polyuria, and an inability to concentrate urine alongside elevated serum osmolality is characteristic of nephrogenic diabetes insipidus. In this condition, the kidneys are unable to respond to antidiuretic hormone (ADH), resulting in the excretion of large volumes of dilute urine. The body's normal response to dehydration, which would involve concentrating urine to retain water, is impaired.

As a result, the serum osmolality increases due to the loss of water, leading to polydipsia as the individual compensates for excessive urination by increasing fluid intake. This aligns with the key features of nephrogenic diabetes insipidus.

The other conditions presented do not fit this symptom profile: central diabetes insipidus involves a lack of ADH production rather than renal resistance to it. SIADH leads to hyponatremia due to excessive water retention and concentrated urine, while hyperaldosteronism primarily affects sodium and potassium balance without leading to the same polydipsia or polyuria patterns. Thus, nephrogenic diabetes insipidus is the correct choice based on these clinical signs and underlying pathophysiology.