**Core Concept**
A left-sided abdominal mass in a baby can be caused by various conditions, including congenital anomalies, tumors, and infections. The differential diagnosis for a pediatric abdominal mass involves considering both benign and malignant etiologies. Accurate diagnosis requires a thorough clinical evaluation, imaging studies, and laboratory tests.

**Why the Correct Answer is Right**
The correct answer depends on the specific case, but common causes of left-sided abdominal masses in babies include neuroblastoma, Wilms tumor, and nephroblastoma. Neuroblastoma is a type of cancer that arises from the neural crest cells and typically presents as a mass in the adrenal gland or sympathetic nervous system. Wilms tumor, also known as nephroblastoma, is a type of kidney cancer that primarily affects children. Both conditions are more common in infants and young children.

**Why Each Wrong Option is Incorrect**
**Option A:** While appendicitis can cause an abdominal mass, it typically presents with symptoms of pain, fever, and leukocytosis, making it less likely to be the primary concern in a case of a left-sided abdominal mass in a baby.
**Option B:** A left-sided hernia can cause a mass, but it is usually associated with a palpable bulge and often occurs in the inguinal region rather than the abdominal cavity.
**Option C:** A left-sided ovarian cyst can occur in infants, but it is relatively rare and typically presents with abdominal distension and vaginal bleeding.
**Option D:** While a left-sided intestinal obstruction can cause abdominal distension and vomiting, it is less likely to present as a discrete mass.

**Clinical Pearl / High-Yield Fact**
In pediatric patients, the presence of a left-sided abdominal mass should prompt a thorough evaluation for neuroblastoma or Wilms tumor, as these conditions are more common in this age group and can have a significant impact on prognosis if not diagnosed promptly.

**Correct Answer:** D.