**Core Concept**
Oseltamivir is an antiviral medication used to treat and prevent influenza A and B infections. It belongs to the class of neuraminidase inhibitors, which work by blocking the release of viral particles from infected cells.

**Why the Correct Answer is Right**
Oseltamivir specifically targets the neuraminidase enzyme, a surface protein of the influenza virus. By inhibiting this enzyme, oseltamivir prevents the virus from spreading to adjacent cells, thereby reducing the severity and duration of the infection. This mechanism is particularly effective against influenza A and B viruses, making oseltamivir a valuable treatment option for these conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because oseltamivir is not a direct-acting antiviral that targets the viral genome or replication machinery.
**Option B:** This option is incorrect because oseltamivir is not an immunomodulator that affects the host immune response to the virus.
**Option C:** This option is incorrect because oseltamivir is not a protease inhibitor, which is a different class of antiviral medication.

**Clinical Pearl / High-Yield Fact**
Oseltamivir is most effective when initiated within 48 hours of symptom onset, as its antiviral effects are most pronounced in the early stages of infection.

**Correct Answer: D. Neuraminidase inhibitor**