**Core Concept**
HBV (Hepatitis B Virus) infectivity is a critical aspect of managing HBV infections, particularly in pregnant women and immunocompromised individuals. The infectivity of HBV is determined by the presence of viral antigens and the immune system's response to the virus.

**Why the Correct Answer is Right**
HBV infectivity is indicated by the presence of the hepatitis B surface antigen (HBsAg) in the blood. HBsAg is a protein on the surface of the virus that triggers an immune response, making the individual contagious. The presence of HBsAg, along with the absence of anti-HBs (antibodies against HBsAg), indicates that the individual is infectious.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a known indicator of HBV infectivity.
**Option B:** This option is incorrect because it does not accurately represent a marker of HBV infectivity.
**Option C:** This option is incorrect because it does not directly relate to HBV infectivity.

**Clinical Pearl / High-Yield Fact**
In pregnant women, HBV infectivity is a significant concern, as vertical transmission (mother-to-child) can occur during delivery. Screening for HBsAg and providing prophylactic treatment can significantly reduce the risk of transmission.

**Correct Answer: D. HBsAg. Presence of Hepatitis B surface antigen (HBsAg) in the blood indicates HBV infectivity.**