**Core Concept**
The excretory portion of the kidney is formed during embryonic development from the intermediate mesoderm, which gives rise to the metanephric kidney. The metanephric kidney is derived from the interaction between two main components: the metanephric mesenchyme (derived from the intermediate mesoderm) and the ureteric bud (derived from the mesonephric duct).

**Why the Correct Answer is Right**
The mesonephric duct, also known as the Wolffian duct, plays a crucial role in the development of the urinary system. During embryonic development, the mesonephric duct gives rise to the ureteric bud, which invades the metanephric mesenchyme and forms the collecting ducts, calyces, and renal pelvis. The ureteric bud also gives rise to the ureters, bladder, and urethra. The metanephric mesenchyme, on the other hand, gives rise to the nephrons, which are the functional units of the kidney responsible for filtration and excretion. The interaction between the ureteric bud and the metanephric mesenchyme is essential for the normal development of the kidney and the formation of the excretory portion.

**Why Each Wrong Option is Incorrect**
* **Option A:** The urogenital sinus is involved in the development of the bladder, urethra, and external genitalia, but it does not contribute to the formation of the excretory portion of the kidney.
* **Option B:** The Mullerian ducts are involved in the development of the female reproductive system, including the fallopian tubes, uterus, and upper part of the vagina, but they do not contribute to the formation of the kidney.
* **Option D:** The genital tubercle gives rise to the external genitalia, but it is not involved in the development of the kidney.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the development of the kidney is a complex process involving the interaction between multiple embryonic structures. Understanding the embryonic development of the kidney is crucial for diagnosing and managing congenital anomalies and kidney diseases.

**✓ Correct Answer: C. Mesonephric duct**