**Core Concept**
Early diagnosis of active hepatitis B infection involves identifying specific markers that indicate current infection. This is crucial for initiating timely treatment and preventing complications. The correct marker should be present in the blood of an individual with active hepatitis B infection.

**Why the Correct Answer is Right**
IgM antibodies to hepatitis B core antigen (HBcAg) are indicative of acute hepatitis B infection. These antibodies are produced in response to the initial exposure to the hepatitis B virus (HBV) and are present in the blood for a short period, typically up to 6 months. The presence of IgM anti-HBcAg is a reliable marker for diagnosing acute hepatitis B infection, as it differentiates it from chronic infection or past exposure. This is because IgM anti-HBcAg is not produced in response to vaccination or chronic infection.

**Why Each Wrong Option is Incorrect**
**Option B:** HBsAg (hepatitis B surface antigen) is a marker of hepatitis B infection, but it can be present in both acute and chronic infections. It does not distinguish between the two.

**Option C:** HBcAg is not directly detectable in the blood due to its location within the hepatitis B virus particle. However, antibodies to HBcAg (anti-HBc) are present in the blood and can be used to diagnose hepatitis B infection.

**Option D:** IgE antibodies to HBsAg are not typically associated with hepatitis B infection. IgE antibodies are more commonly involved in allergic reactions.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the presence of IgM anti-HBcAg is a strong indicator of acute hepatitis B infection, and its absence or presence of IgG anti-HBcAg suggests chronic infection or past exposure.

**✓ Correct Answer: A. IgM HBcAg antibody**