**Core Concept**
HBV (Hepatitis B virus) infection is a significant public health concern, and early detection is crucial for preventing liver disease progression. The presence of HBsAg (hepatitis B surface antigen) and HBeAg (hepatitis B e-antigen) indicates active viral replication and infectivity. Liver enzymes, such as SGOT (serum glutamic-oxaloacetic transaminase) and SGPT (serum glutamic-pyruvic transaminase), are often elevated in acute or chronic hepatitis B, but normal levels do not rule out infection.

**Why the Correct Answer is Right**
HBV-DNA estimation is a crucial step in managing HBV infection. It measures the viral load, which helps in assessing the severity of infection and guiding treatment. In patients with normal liver enzymes, HBV-DNA estimation is essential to identify those with high viral loads, who may be at risk of liver disease progression. This test also helps in monitoring the effectiveness of antiviral therapy and making informed decisions about treatment duration.

**Why Each Wrong Option is Incorrect**

**Option A:** Observation is not sufficient in this case, as it may lead to delayed diagnosis and treatment of chronic HBV infection, potentially resulting in liver damage and cirrhosis.

**Option B:** Liver biopsy is an invasive procedure and may not be necessary in patients with normal liver enzymes. It is usually reserved for cases with abnormal liver function tests or when the diagnosis is uncertain.

**Option C:** Interferon therapy is an older treatment option for HBV infection, but its use has declined with the availability of more effective and tolerable antiviral medications. It may not be the best initial choice without assessing the viral load.

**Clinical Pearl / High-Yield Fact**
In patients with chronic HBV infection, the presence of HBsAg and HBeAg indicates active viral replication, whereas the absence of HBeAg suggests a lower viral load and reduced infectivity.

**✓ Correct Answer: D. HBV-DNA estimation**