**Question:** All/except are true about Multiple Sclerosis (MS):

A. MS is an autoimmune disorder where the immune system attacks the central nervous system (CNS).
B. MS primarily affects children.
C. MS is caused by a deficiency in vitamin D.
D. MS is a rare condition affecting less than 1% of the population.

**Core Concept:** Multiple Sclerosis (MS) is a chronic, inflammatory and autoimmune disease that primarily affects the central nervous system (CNS), including the brain, spinal cord, and optic nerves. In MS, the immune system mistakenly attacks the myelin sheath, which is the fatty substance that covers and protects the axons (nerve fibers) in the CNS, causing scarring and disrupting the normal function of these neurons.

**Why the Correct Answer is Right:**
A. MS is an autoimmune disorder where the immune system attacks the central nervous system (CNS). This is accurate because the disease is characterized by an immune system dysfunction leading to inflammation and demyelination in the CNS.

**Why Each Wrong Option is Incorrect:**
B. MS primarily affects children is incorrect because MS typically develops in young adults (onset between 20-40 years) and its incidence increases with age.
C. MS is caused by a deficiency in vitamin D is incorrect because the exact cause of MS is unknown and complex, involving genetic, environmental, and lifestyle factors. While vitamin D deficiency has been linked to MS risk, it is not the sole cause.
D. MS is a rare condition affecting less than 1% of the population is incorrect as the prevalence of MS varies across different populations, but it is estimated to affect about 0.1% to 2% of the global population.

**Clinical Pearl:**
MS is a complex and multifactorial disease, involving genetic, environmental, and lifestyle factors. While vitamin D deficiency has been associated with an increased risk of MS, it is not the sole cause. The disease primarily affects young adults and its prevalence varies across populations.

**Correct Answer:**
D. MS is a rare condition affecting less than 1% of the population

**Explanation:**
MS is not a rare condition, as it affects approximately 0.1% to 2% of the global population. However, the prevalence varies across different populations and regions, making it neither extremely rare nor extremely common. The focus on MS being a rare condition could be misleading and may lead to an overestimation or underestimation of the prevalence in certain populations.