**Core Concept**
Pyogenic liver abscess is a collection of pus in the liver, usually caused by bacterial infection. The common causes involve the spread of infection from other parts of the body, such as the biliary tract, or the direct introduction of bacteria into the liver through the bloodstream.

**Why the Correct Answer is Right**
The most common cause of pyogenic liver abscess is biliary tract disease, specifically cholangitis, which is an infection of the bile ducts. This is due to the direct spread of bacteria from the bile ducts into the liver parenchyma. The bacteria most commonly involved in cholangitis are Escherichia coli (E. coli), Klebsiella pneumoniae, and Streptococcus pneumoniae.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the bloodstream can be a route for bacterial dissemination, it is not the most common source of infection for liver abscesses.
* **Option B:** Although gallstones can cause obstruction and increase the risk of cholangitis, they are not the primary cause of pyogenic liver abscess.
* **Option C:** Fungal liver abscesses are less common than pyogenic abscesses and typically occur in immunocompromised individuals.

**Clinical Pearl / High-Yield Fact**
In areas where Klebsiella pneumoniae is prevalent, such as in East Asia, it is essential to consider this pathogen as a primary cause of pyogenic liver abscess, especially in patients with diabetes mellitus.

**Correct Answer: B. Biliary tract disease, specifically cholangitis.**