Which term describes urine with osmolality similar to plasma, indicating reduced concentrating ability often observed in renal disease?

• A. Hyposthenuria
• B. Hypersthenuria
• C. Hyperosmolar urine
• D. Isosthenuria

Answer: (D)

Isosthenuria is the pattern you see when the kidney loses its ability to concentrate or dilute urine. In this state, the urine osmolality sits about the same as plasma (roughly the same 300 mOsm/kg range). That happens with renal disease or tubular/medullary dysfunction, where the kidney can’t establish the usual osmotic gradient or respond properly to ADH. In healthy kidneys, urine can be much more concentrated than plasma when hydrated restricts water or much more dilute when excess water is present. So urine that matches plasma indicates impaired concentrating capacity. The other terms describe urine that is either more dilute or more concentrated than plasma, which is not what’s described here.