**Core Concept**
The question is testing the student's ability to diagnose a condition characterized by massive splenomegaly without hepatomegaly, leading to massive hematemesis in a child. This condition is often associated with an underlying hematological disorder.

**Why the Correct Answer is Right**
The massive splenomegaly without hepatomegaly suggests a condition that primarily affects the spleen. In children, the most common cause of such presentation is **Hereditary Spherocytosis (HS)**. HS is a genetic disorder characterized by the production of abnormal red blood cells that are prone to hemolysis. The spleen becomes enlarged as it tries to filter out these defective red blood cells, leading to massive splenomegaly. Over time, the repeated destruction of red blood cells can lead to iron overload, which may cause gastric mucosal damage and subsequent hematemesis.

**Why Each Wrong Option is Incorrect**
**Option A:** While portal hypertension can cause splenomegaly, it typically leads to hepatomegaly as well due to congestion in the liver.

**Option B:** Leukemia can cause splenomegaly, but it is usually accompanied by other systemic symptoms such as fever, weight loss, and bone pain.

**Option C:** Thalassemia major can cause splenomegaly due to hemolysis, but it is typically accompanied by other symptoms such as anemia, jaundice, and bone deformities.

**Clinical Pearl / High-Yield Fact**
Hereditary Spherocytosis is a common cause of massive splenomegaly in children, and it is essential to consider this diagnosis in any child presenting with massive hematemesis and splenomegaly without hepatomegaly.

**Correct Answer:** C. Thalassemia major