**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination of the central nervous system (CNS) due to an immune-mediated attack on the myelin sheath. The disease is classified into four main types based on the course of the disease.

**Why the Correct Answer is Right**
The most common type of multiple sclerosis is relapsing-remitting multiple sclerosis (RRMS), accounting for approximately 85% of all MS cases. RRMS is characterized by recurring episodes of neurological symptoms followed by periods of partial or complete recovery. The relapses are thought to be triggered by the immune system attacking the myelin sheath, leading to inflammation and damage to the CNS. The exact cause of relapses is not fully understood, but it is believed to be related to the activation of autoreactive T cells and the production of pro-inflammatory cytokines.

**Why Each Wrong Option is Incorrect**
**Option A:** Secondary progressive multiple sclerosis (SPMS) is a less common type of MS that accounts for approximately 10-15% of cases. It is characterized by a steady worsening of symptoms without distinct relapses.
**Option B:** Primary progressive multiple sclerosis (PPMS) is a rare type of MS that accounts for approximately 5-10% of cases. It is characterized by a steady worsening of symptoms from the onset without distinct relapses.
**Option C:** Progressive-relapsing multiple sclerosis (PRMS) is a rare type of MS that accounts for approximately 2-5% of cases. It is characterized by a steady worsening of symptoms from the onset, with distinct relapses.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between the various types of MS, as each type has a distinct prognosis and treatment approach. A thorough history and physical examination, along with diagnostic imaging and laboratory tests, can help determine the type of MS.

**Correct Answer:** . Relapsing-remitting multiple sclerosis (RRMS).