**Core Concept**
The question is testing the understanding of the epidemiology of hepatitis viruses transmitted through blood transfusion. Hepatitis B virus (HBV) was the most common cause of post-transfusion hepatitis, but with the implementation of screening tests, its incidence has decreased. The next most common cause is another hepatitis virus that can be transmitted through blood transfusion.

**Why the Correct Answer is Right**
The correct answer is hepatitis C virus (HCV). HCV is a flavivirus that can be transmitted through blood transfusion, and its prevalence has increased significantly since the implementation of HBV screening. HCV replicates in the liver and can cause chronic infection, leading to liver cirrhosis and hepatocellular carcinoma. The virus enters the body through the bloodstream and infects hepatocytes, where it replicates and causes liver damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Hepatitis A virus (HAV) is primarily transmitted through the fecal-oral route and is not commonly transmitted through blood transfusion.

**Option B:** Hepatitis D virus (HDV) requires the presence of HBV to replicate and is not the next most common cause of post-transfusion hepatitis after HBV.

**Option D:** Hepatitis E virus (HEV) is also primarily transmitted through the fecal-oral route and is not commonly transmitted through blood transfusion.

**Clinical Pearl / High-Yield Fact**
Hepatitis C virus is a major cause of post-transfusion hepatitis, and its transmission can be prevented by screening blood donations for HCV antibodies.

**Correct Answer:** C. Hepatitis C virus.