## Why option 1 is correct The structure marked **A** (portal vein) normally maintains a hepatic venous pressure gradient (HVPG) < 5 mmHg. In cirrhosis, intrahepatic resistance increases, raising portal pressure. When HVPG exceeds 10 mmHg, varices develop; at ≥ 12 mmHg, variceal bleeding risk becomes high. The elevated pressure in the portal vein forces blood to seek alternative routes through porto-systemic collaterals at sites of embryologic anastomosis (esophageal, gastric, anorectal, caput medusae), causing variceal dilation. This is the fundamental mechanism of variceal formation in portal hypertension (Gray's Anatomy 42e Ch 65; Harrison 21e Ch 339). ## Why each distractor is wrong - **Option 2**: While cirrhosis does reduce hepatic artery flow, the hepatic artery provides only ~30% of hepatic blood flow; the portal vein provides ~70%. Varices form due to elevated portal pressure, not hepatic artery insufficiency. Hepatic artery changes do not directly cause variceal formation. - **Option 3**: Splenic vein thrombosis causes isolated gastric varices ("left-sided portal hypertension"), not esophageal varices. This patient has esophageal varices from cirrhosis, which is a hepatic (intrahepatic) cause of portal hypertension, not pre-hepatic splenic vein disease. - **Option 4**: Portal hypertension does not increase cardiac output; it increases resistance. Varices form due to pressure gradient reversal and collateral opening, not mechanical stress from increased flow. This option misrepresents the hemodynamic mechanism. **High-Yield:** Portal hypertension varices develop when HVPG ≥ 10 mmHg; clinically significant bleeding risk at ≥ 12 mmHg. Non-selective β-blockers reduce portal pressure and prevent variceal bleeding. [cite: Gray's Anatomy 42e Ch 65; Harrison 21e Ch 339]
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