Isosthenuria is defined by a series of urine specimens from the same patient showing what characteristic?

• A. Specific gravity of exactly 1.000
• B. Specific gravity less than 1.007
• C. Specific gravity greater than 1.020
• D. Fixed specific gravity of approximately 1.010

Answer: (D)

Isosthenuria shows the kidneys’ inability to modify urine concentration. When the tubules can’t concentrate or dilute urine, the urine stays at a constant osmolality similar to plasma, reflected by a specific gravity around 1.010. This fixed value persists across different samples from the same patient, regardless of hydration changes, signaling that the kidneys have lost their concentrating and diluting capacity. That’s why the description of a fixed specific gravity of approximately 1.010 best matches isosthenuria.

Why the other patterns don’t fit: a specific gravity of 1.000 is essentially water and would indicate extremely dilute urine, not the steady ~1.010 seen in isosthenuria. A value less than 1.007 is also dilute and not a fixed, unchanged SG around 1.010. A value greater than 1.020 shows concentrated urine, which would reflect intact or enhanced concentrating ability under some conditions, not the inability described by isosthenuria.