**Core Concept**
Pyogenic liver abscesses most commonly arise from bacterial infection spreading via the biliary system, particularly from cholangitis or gallbladder disease. The biliary tree serves as a major portal for bacterial entry into the liver, leading to infection and abscess formation.

**Why the Correct Answer is Right**
The most common route of pyogenic liver abscess is **ascending infection through the biliary tract**, especially in patients with cholelithiasis or bile duct obstruction. Bacteria such as *E. coli*, *Enterococcus*, and *Klebsiella* ascend from the gut into the bile ducts, causing cholangitis. This infection then spreads to the liver parenchyma, leading to abscess formation. The biliary system provides a direct anatomical pathway from the gastrointestinal tract to the liver, making it the primary source of infection in most cases.

**Why Each Wrong Option is Incorrect**
Option A: Hematogenous spread through portal vein is a **less common** route and typically occurs in systemic infections (e.g., sepsis), not the most frequent cause.
Option C: Hepatic artery involvement is not a route of spread but rather a source of blood supply; bacteria do not travel via hepatic artery.
Option D: Local spread is rare and usually seen in cases of direct trauma or adjacent organ infection, not the most common mechanism.

**Clinical Pearl / High-Yield Fact**
In pyogenic liver abscess, **biliary obstruction is the most common precipitant**, and **ascending cholangitis** is the leading cause. Always consider gallstones or biliary strictures in patients presenting with fever and right upper quadrant pain.

✓ Correct Answer: B. Ascending infection through biliary tract