**Core Concept**
The question is testing the knowledge of the most common cause of hepatitis in children, specifically in the pediatric population. Hepatitis refers to inflammation of the liver, which can be caused by various factors including viral infections, toxins, and autoimmune disorders.

**Why the Correct Answer is Right**
In children, especially in the age group of 3 years, the most common cause of hepatitis is Hepatitis A virus (HAV). HAV is a highly contagious virus that is primarily spread through the fecal-oral route, often through contaminated food or water, or through close contact with an infected person. The virus replicates in the liver, leading to inflammation and damage to liver cells. The clinical presentation of HAV infection typically includes symptoms such as fever, fatigue, loss of appetite, nausea, vomiting, abdominal pain, and jaundice.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hepatitis B virus (HBV) is a significant cause of hepatitis in children, but it is more commonly associated with vertical transmission from mother to child or through perinatal exposure. In contrast, HAV is more commonly spread through fecal-oral contamination.
* **Option B:** Hepatitis C virus (HCV) is a significant cause of chronic hepatitis, but it is less commonly associated with acute hepatitis in children. HCV is typically spread through parenteral exposure, such as through blood transfusions or needle sharing.
* **Option C:** Autoimmune hepatitis is a rare cause of hepatitis in children, and it typically presents with a more chronic course of disease. It is not the most common cause of hepatitis in children.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the most common cause of hepatitis in children can vary depending on the region and the population being studied. However, in general, HAV is a significant public health concern, particularly in areas with poor sanitation and hygiene.

**Correct Answer: A. Hepatitis A virus (HAV)**