**Core Concept**
Hepatitis A vaccine is a recombinant vaccine that induces immunity against the hepatitis A virus (HAV), a major cause of acute hepatitis worldwide. The vaccine is typically administered to prevent infection in individuals at high risk of exposure, such as travelers to endemic areas, individuals with certain medical conditions, and those with occupational exposure.

**Why the Correct Answer is Right**
The hepatitis A vaccine is usually recommended for individuals aged 2 years and older, with a booster dose given 6-12 months later. This schedule is designed to provide long-term immunity against HAV. The vaccine is typically administered in a series of two doses, with the second dose given 6-12 months after the first dose. This schedule is based on the immune response elicited by the vaccine, which is known to provide protection against HAV for at least 20 years.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the hepatitis A vaccine is not typically recommended for infants younger than 2 years old, as they are protected by maternal antibodies acquired during pregnancy.
**Option B:** This option is incorrect because the hepatitis A vaccine is not usually given as a single dose; a booster dose is required to ensure long-term immunity.
**Option C:** This option is incorrect because the hepatitis A vaccine is not typically recommended for individuals older than 50 years old, unless they have specific risk factors for HAV infection.

**Clinical Pearl / High-Yield Fact**
The hepatitis A vaccine is a safe and effective way to prevent HAV infection, which can cause severe liver disease and even death in some cases. It is essential to follow the recommended vaccination schedule to ensure optimal protection against HAV.

**Correct Answer:** D. 2 years and older