**Core Concept**
Neuroblastoma is a type of malignant tumor that arises from the embryonal cells of the sympathetic nervous system. It is the most common extracranial solid tumor in children, often presenting as an abdominal mass.

**Why the Correct Answer is Right**
Neuroblastoma typically originates in the adrenal glands, although it can also arise in the neck, chest, or pelvis. The tumor cells secrete catecholamines, which can lead to symptoms such as hypertension, tachycardia, and flushing. Imaging studies like ultrasound, CT, or MRI can reveal the characteristic imaging features of neuroblastoma, including a large abdominal mass with calcifications. The tumor often metastasizes to the lymph nodes, liver, and bone marrow.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydronephrosis refers to the swelling of the kidney due to the accumulation of urine, often caused by obstruction of the urinary tract. It is not a type of abdominal mass.

**Option B:** Wilms' tumor is a type of renal cancer that primarily affects children, but it is less common than neuroblastoma. Wilms' tumor typically arises in the kidney and presents as an abdominal mass.

**Option D:** Rhabdomyosarcoma is a type of soft tissue sarcoma that can occur in various locations, including the orbit, genitourinary tract, or extremities. While it can present as an abdominal mass, it is not the most common type of abdominal mass in children.

**Clinical Pearl / High-Yield Fact**
Neuroblastoma is often associated with a poor prognosis if it presents with metastatic disease. However, children with localized neuroblastoma have a significantly better outcome with prompt treatment.

**✓ Correct Answer: C. Neuroblastoma**