**Core Concept**
HBV infection has a complex serological profile, with various markers appearing at different stages of the infection. The "window phase" refers to the period between the disappearance of HBsAg (hepatitis B surface antigen) and the appearance of anti-HBs (antibody to HBsAg), where the only detectable marker may be anti-HBc (antibody to hepatitis B core antigen).

**Why the Correct Answer is Right**
During the window phase, the patient has cleared HBsAg but has not yet developed anti-HBs. However, they still have anti-HBc, which is produced in response to the core antigen. This core antigen is not part of the virus itself but is released from the virus during its replication cycle. The presence of anti-HBc during this phase is crucial for diagnosing HBV infection, as it indicates that the patient has been exposed to the virus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but typically, it might be a distractor like "HBsAg" or "anti-HBs". These markers are not the only detectable serological markers during the window phase.
* **Option B:** This option is not provided, but it might be a distractor like "anti-HCV" (antibody to hepatitis C virus). This is incorrect because anti-HCV is not related to HBV infection.
* **Option D:** This option is not provided, but it might be a distractor like "HBeAg" (hepatitis B e-antigen). While HBeAg is a marker of viral replication, it is not the only detectable serological marker during the window phase.

**Clinical Pearl / High-Yield Fact**
The window phase is a critical period in diagnosing HBV infection, and the presence of anti-HBc is essential for identifying patients who have been exposed to the virus. This knowledge is crucial for clinicians to provide appropriate management and follow-up for patients with HBV infection.

**Correct Answer: C. anti-HBc**