**Low and fixed specific gravity of urine is seen in Diabetes Mellitus.**

**Core Concept**
Diabetes Mellitus, particularly type 1, leads to a condition known as nephrogenic diabetes insipidus due to the body's inability to respond to antidiuretic hormone (ADH). This results in an inability to concentrate urine, causing a consistently low and fixed specific gravity.

**Why the Correct Answer is Right**
In diabetes mellitus, the kidneys are unable to respond to ADH, leading to a failure to reabsorb water in the collecting ducts. This results in an excessive loss of water in the urine, causing a decrease in the specific gravity. The specific gravity remains low and fixed due to the constant production of large volumes of dilute urine. The underlying pathophysiology involves the disruption of the normal feedback loop between the kidneys and the posterior pituitary gland, leading to an inability to regulate water reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the pathophysiology of diabetes mellitus. While certain medications can cause changes in urine specific gravity, this is not a characteristic of diabetes mellitus.
**Option B:** This option is incorrect because it is not a condition that affects the kidneys' ability to concentrate urine. Instead, it is a condition that affects the production of urine, not its concentration.
**Option C:** This option is incorrect because it does not directly relate to the pathophysiology of diabetes mellitus. While certain conditions can cause changes in urine specific gravity, this is not a characteristic of diabetes mellitus.

**Clinical Pearl / High-Yield Fact**
Nephrogenic diabetes insipidus, a condition caused by the kidneys' inability to respond to ADH, is a classic exam trap. Students should remember that a low and fixed specific gravity of urine is a hallmark of this condition and is often seen in patients with diabetes mellitus.

**Correct Answer: D. Diabetes Mellitus.**