What is the likely diagnosis for a dog with a USG of 1.005 that does not respond to water deprivation?

A urine specific gravity (USG) of 1.005 indicates that the dog is producing very dilute urine, which suggests a problem with the kidneys' ability to concentrate urine. In the case of nephrogenic diabetes insipidus, the kidneys are unable to respond to antidiuretic hormone (ADH), even though the hormone is present and normally functions in healthy animals.

When faced with water deprivation, a healthy dog's kidneys should concentrate urine and produce a higher USG. However, if the kidneys are not responding to ADH due to a receptor or aquaporin deficiency, the USG remains low despite efforts to encourage concentration through dehydration. Thus, nephrogenic diabetes insipidus is characterized by dilute urine and an inability to concentrate that urine under water deprivation conditions.

Other conditions like central diabetes insipidus involve an issue with ADH production rather than kidney response; chronic kidney disease may feature a concentrated, but later on may have dilute urine due to advanced kidney failure; and acute renal failure often shows more severe changes in urine concentration. Therefore, the USG and response to water deprivation distinctly point towards nephrogenic diabetes insipidus as the likely diagnosis in this scenario.