## **Core Concept**
The question tests knowledge of viral envelope proteins, specifically focusing on **hemagglutinin**, **neuraminidase**, and **membrane fusion proteins**. These proteins are crucial for the infectivity and replication of enveloped viruses. Hemagglutinin and neuraminidase are characteristic surface proteins of influenza viruses, while membrane fusion proteins enable viral entry into host cells.

## **Why the Correct Answer is Right**
The correct answer, **Respiratory Syncytial Virus (RSV)**, is a type of enveloped virus that causes respiratory infections. RSV lacks **hemagglutinin** and **neuraminidase** but possesses a **membrane fusion protein** (F protein) essential for its entry into host cells. The F protein facilitates the fusion of the viral envelope with the host cell membrane, making it critical for the virus's lifecycle.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Influenza virus has **hemagglutinin** and **neuraminidase**, making it incorrect.
- **Option B:** HIV (Human Immunodeficiency Virus) has envelope proteins like gp120 and gp41, which mediate entry but does not primarily lack hemagglutinin and neuraminidase in the context implied here; it is not characterized by these terms.
- **Option C:** Herpes simplex virus has **envelope glycoproteins** that facilitate entry but is not specifically known for lacking hemagglutinin and neuraminidase in a way that matches the question's context.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **RSV** primarily affects young children and can cause severe lower respiratory tract infections. The **F protein** of RSV is a target for the development of vaccines and monoclonal antibodies, such as palivizumab, used to prevent severe RSV disease in high-risk infants.

## **Correct Answer:** .