**Core Concept**
The question is testing the ability to diagnose a condition causing esophageal varices, which are dilated submucosal veins in the esophagus. This condition is typically associated with portal hypertension, which is increased pressure in the portal vein system.

**Why the Correct Answer is Right**
The presence of esophageal varices in a young boy with mild splenomegaly suggests portal hypertension. The most common cause of portal hypertension in children is non-cirrhotic portal fibrosis (NCPF), also known as non-cirrhotic portal hypertension. NCPF is a condition where there is fibrosis of the portal vein branches without underlying liver cirrhosis. This leads to increased resistance to blood flow in the portal vein, resulting in portal hypertension and subsequent esophageal varices.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cirrhosis is a common cause of portal hypertension in adults, but it is less common in children. In cirrhosis, there is scarring of the liver (cirrhosis) that leads to portal hypertension.
* **Option B:** Extrahepatic portal vein thrombosis (EHPVT) is a condition where there is a blood clot in the portal vein outside the liver. While this can cause portal hypertension, it is less likely to present with esophageal varices in a young boy.
* **Option C:** Hepatic schistosomiasis is a parasitic infection that can cause portal hypertension, but it is not a common cause in children.

**Clinical Pearl / High-Yield Fact**
Be aware that non-cirrhotic portal fibrosis (NCPF) is a common cause of portal hypertension in children, and it can present with esophageal varices without underlying liver cirrhosis.

**Correct Answer:** D.