**Core Concept:** Polyuria refers to the increased production of urine due to an increase in the rate of urine formation or a decrease in urine concentration. Specific gravity is a measure of the concentration of a solution, with higher values indicating a more concentrated solution. Low fixed specific gravity urine suggests a decreased concentration ability of the kidneys, which may indicate impaired kidney function.

**Why the Correct Answer is Right:** Polyuria with low fixed specific gravity urine is typically seen in conditions involving impaired kidney function, such as diabetes insipidus. In diabetes insipidus, the kidneys are unable to concentrate urine effectively due to the deficient production or action of antidiuretic hormone (ADH) or vasopressin. ADH acts on the collecting ducts in the nephron to increase the permeability of water channels, leading to increased reabsorption of water and thus concentrating the urine.

**Why Each Wrong Option is Incorrect:**

A. Polyuria with high fixed specific gravity urine is more commonly associated with conditions like diabetes mellitus or renal tubular acidosis, where excessive urine production is due to increased water loss from the body or impaired acid-base balance, respectively.
B. Polyuria with variable specific gravity urine can be seen in conditions like diabetes mellitus, where urine concentration ability is impaired, but the specific gravity can vary due to fluctuations in water excretion.
C. Polyuria with high fixed specific gravity urine is not the correct answer, as it would be associated with conditions like diabetes mellitus, where increased urine production is due to high blood glucose levels and increased water excretion.
D. Polyuria with normal specific gravity urine is usually observed in conditions like hyperthyroidism, where increased urine production is due to the increased kidney blood flow and filtration rate.

**Clinical Pearl:** A low fixed specific gravity urine along with polyuria is a clinical clue to the diagnosis of diabetes insipidus, prompting further evaluation to confirm the diagnosis and initiate appropriate management.