**Core Concept**
The patient's presentation of massive hematemesis with a history of fever and splenomegaly suggests an underlying liver disease leading to portal hypertension, which causes the formation of esophageal varices.

**Why the Correct Answer is Right**
The patient's symptoms and clinical findings are consistent with a diagnosis of portal hypertension, which can be caused by chronic liver disease such as hepatitis or cirrhosis. The formation of esophageal varices is a complication of portal hypertension, where blood flow is redirected through collateral vessels in the esophagus, increasing the risk of bleeding. The history of fever for the past 14 days, which was managed with drugs, may indicate a liver infection such as viral hepatitis, leading to liver damage and subsequent portal hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** NSAID-induced duodenal ulcer is less likely in this scenario, as the patient's symptoms and clinical findings do not suggest a typical peptic ulcer disease. NSAID use would not explain the splenomegaly or the history of fever.
**Option B:** Drug-induced gastritis is also unlikely, as the patient's symptoms and clinical findings are more suggestive of a liver-related condition rather than a direct effect of medication on the stomach.
**Option D:** This option is incorrect because one of the above options (C) is indeed the correct diagnosis.

**Clinical Pearl / High-Yield Fact**
In a patient with a history of fever and splenomegaly, always consider the possibility of underlying liver disease, including viral hepatitis, which can lead to portal hypertension and the formation of esophageal varices.

**✓ Correct Answer: C. Esophageal varices**