**Core Concept**
The question is testing knowledge of congenital anomalies of the upper urogenital tract, specifically the most common type. The upper urogenital tract refers to the kidneys and ureters.

**Why the Correct Answer is Right**
The most common anomaly of the upper urogenital tract is a congenital defect of the kidney or ureter. Specifically, an ectopic kidney or a duplex kidney system is a common congenital anomaly. However, the most common type of anomaly is a **renal pelvis duplication**. This condition occurs when there are two renal pelvises, each connected to a separate ureter, instead of one single renal pelvis connected to a single ureter. This anomaly can be associated with other anomalies such as a horseshoe kidney or a crossed fused ectopic kidney.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a specific type of upper urogenital tract anomaly. The question is asking for the most common type of anomaly, not a general category.
**Option B:** This option is incorrect because it is a type of lower urogenital tract anomaly, specifically a congenital defect of the bladder or urethra.
**Option C:** This option is incorrect because it is not a specific type of upper urogenital tract anomaly. The question is asking for a specific type of anomaly, not a general category.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the most common type of upper urogenital tract anomaly is a renal pelvis duplication. This condition can be associated with other anomalies such as a horseshoe kidney or a crossed fused ectopic kidney. A high index of suspicion is required to diagnose this condition, as it can lead to complications such as urinary tract infections and kidney stones.

**Correct Answer: C. Renal pelvis duplication.**