**Core Concept**
Acute pyelonephritis is a bacterial infection of the renal parenchyma, characterized by inflammation of the renal cortex and medulla. The infection can lead to the presence of white blood cells (WBCs) in the urine, which is a key diagnostic finding.

**Why the Correct Answer is Right**
The presence of WBC casts in the urine is a specific indicator of pyelonephritis. WBC casts are formed when WBCs, which have been filtered by the glomerulus, are reabsorbed and then excreted by the renal tubules. This process occurs in the kidneys, indicating that the infection is localized to the renal parenchyma. The presence of WBC casts is more specific than other urinary findings, such as WBCs or bacteria, as it suggests that the infection has spread to the renal interstitium.

**Why Each Wrong Option is Incorrect**
**Option B:** Leucocyte esterase test is a screening test for the presence of WBCs in the urine, but it is not specific for pyelonephritis. It can be positive in other urinary tract infections as well.
**Option C:** Nitrite test is positive in the presence of bacterial infection, but it is not specific for pyelonephritis. Many bacteria do not produce nitrite, and this test can be false-negative in cases of pyelonephritis caused by these organisms.
**Option D:** Bacteria in gram stain can be present in both pyelonephritis and lower urinary tract infections. The presence of bacteria in the urine is not specific for pyelonephritis and can be seen in other types of urinary tract infections.

**Clinical Pearl / High-Yield Fact**
WBC casts are a specific indicator of pyelonephritis and should prompt further investigation, including imaging studies and antibiotic sensitivity testing, to confirm the diagnosis.

**✓ Correct Answer: A. W.B.C. casts**