**Question:** Cobra head sign on IVP is classically seen with

A. Kidney stone
B. Pyelonephritis
C. Hydronephrosis
D. Ureteric calculi

**Correct Answer:** **D. Ureteric calculi**

**Core Concept:** The cobra head sign is a radiological finding observed during intravenous pyelography (IVP), a procedure that involves injecting a contrast medium into the vein to visualize the nephrons, ureters, and bladder. This sign is distinctive and can help identify specific pathological conditions affecting the urinary system.

**Why the Correct Answer is Right:** The cobra head sign is associated with ureteric calculi (stones) due to the characteristic appearance of the obstructed ureter. When a calculus obstructs the ureter, the contrast medium within the obstructed ureter appears as a double-headed arrowhead or a "cobra head" sign on IVP. This sign is not seen with kidney stone, pyelonephritis, or hydronephrosis, as their respective clinical presentations and imaging findings are distinct.

**Why Each Wrong Option is Incorrect:**

A. Kidney stone (nephrolithiasis): Although kidney stones can cause obstructive uropathy, the cobra head sign is not a specific feature of kidney stones. The clinical presentation, imaging findings, and management strategies differ between ureteric calculi and kidney stones.

B. Pyelonephritis: Pyelonephritis is an infection, causing inflammation rather than obstruction. The imaging findings in pyelonephritis might include hydronephrosis but not the cobra head sign.

C. Hydronephrosis: Hydronephrosis refers to the dilation of the renal pelvis and ureter due to obstruction. The cobra head sign is not a feature of hydronephrosis. Hydronephrosis can be caused by various conditions, but the imaging findings may not specifically indicate ureteric calculi.

**Clinical Pearl:**

The cobra head sign is a valuable clue to diagnose ureteric calculi on IVP, as it helps differentiate it from other urinary system disorders. Ureteric calculi can cause a specific radiological finding, while the other options generally present with distinct clinical features and imaging findings. A comprehensive clinical evaluation and correlation with imaging findings are essential for accurate diagnosis and management of patients with suspected obstructive uropathy.