**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination, axonal loss, and neurodegeneration in the central nervous system (CNS). The relapsing-remitting type of MS is the most common form, where patients experience periods of relapses or exacerbations followed by periods of partial or complete recovery. The goal of treatment is to reduce the frequency and severity of relapses, slow disease progression, and improve quality of life.

**Why the Correct Answer is Right**
Beta interferon (Beta IFN) is the drug of choice for relapsing-remitting multiple sclerosis (RRMS). Beta IFN works by modulating the immune system, reducing inflammation, and preventing demyelination. It binds to the interferon-gamma receptor, which inhibits the production of pro-inflammatory cytokines and promotes the production of anti-inflammatory cytokines. This leads to a reduction in the frequency and severity of relapses, as well as a slowing of disease progression. Beta IFN is administered via subcutaneous injection and is available in various formulations, including Avonex, Rebif, and Betaseron.

**Why Each Wrong Option is Incorrect**
**Option A:** Alpha IFN is primarily used to treat viral infections, such as hepatitis C, and certain types of cancer. It is not effective in treating multiple sclerosis.
**Option C:** Gamma IFN is a pro-inflammatory cytokine that exacerbates autoimmune diseases, including multiple sclerosis. It is not used to treat RRMS.
**Option D:** Natalizumab is a monoclonal antibody that targets the integrin alpha-4 subunit, preventing the migration of immune cells into the CNS. While it is effective in treating RRMS, it is not the first-line treatment and is typically reserved for patients who have failed other therapies.

**Clinical Pearl / High-Yield Fact**
Beta IFN can cause flu-like symptoms, injection site reactions, and increased liver enzymes. Patients should be monitored for these side effects and have regular blood tests to assess liver function.

**✓ Correct Answer: B. Beta IFN**