**Core Concept**
Broad casts in urinary sediments are indicative of an underlying renal pathology, specifically related to the distal convoluted tubule and collecting ducts. This finding is associated with impaired renal function, often as a result of chronic conditions such as diabetes mellitus, hypertension, or chronic kidney disease.

**Why the Correct Answer is Right**
Broad casts are typically seen in conditions where there is increased pressure within the renal tubules, leading to the stretching and dilation of these structures. This results in the formation of broad, flat casts that are visible under microscopic examination. The presence of broad casts suggests that the renal tubules are unable to reabsorb or concentrate urine effectively, indicating a significant impairment in renal function.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because narrow casts are typically associated with conditions such as acute tubular necrosis or tubular obstruction, and are not specific for the same underlying pathology as broad casts.

**Option B:** This is incorrect because red blood cells in the urine are indicative of hematuria, which can be caused by a variety of conditions including urinary tract infections, kidney stones, or glomerulonephritis.

**Option C:** This is incorrect because white blood cells in the urine are indicative of pyuria, which is often associated with urinary tract infections or other inflammatory conditions.

**Clinical Pearl / High-Yield Fact**
Broad casts in urinary sediments are often seen in conjunction with proteinuria, which can be an important diagnostic clue for conditions such as diabetic nephropathy or nephrotic syndrome.

**Correct Answer: D.**