**Core Concept**
The hepatitis A vaccine is a crucial component of preventive medicine, particularly for individuals traveling to areas with high endemicity of hepatitis A virus (HAV) infection. The vaccine's efficacy is influenced by the schedule and number of doses administered.

**Why the Correct Answer is Right**
The standard schedule for hepatitis A vaccination involves administering a single dose of inactivated HAV vaccine to adults aged 18 years and older. This schedule is recommended by the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO). The vaccine induces immunity against HAV, preventing infection and its associated complications. The immune response is mediated by the production of antibodies against HAV, which are essential for neutralizing the virus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the hepatitis A vaccine schedule typically involves a single dose, not multiple doses. While some vaccines may require a booster dose, hepatitis A vaccination does not.
* **Option B:** This option is incorrect because the hepatitis A vaccine is not administered concurrently with other vaccines, such as the hepatitis B vaccine. While co-administration may be feasible, it is not the standard practice.
* **Option C:** This option is incorrect because the hepatitis A vaccine is not administered to children under the age of 12 months. The vaccine is approved for use in individuals aged 2 years and older.

**Clinical Pearl / High-Yield Fact**
It is essential to note that individuals with pre-existing liver disease or immunocompromising conditions should receive hepatitis A vaccination to prevent severe outcomes. Additionally, travelers to areas with high endemicity of HAV infection should receive vaccination at least 2 weeks prior to departure to ensure adequate immunity.

**Correct Answer: D.**