**Core Concept**
The normal distribution (Gaussian curve) is a fundamental concept in statistics and public health, describing how data points are distributed around a mean. In a normal distribution, approximately 68% of values fall within one standard deviation of the mean.

**Why the Correct Answer is Right**
In a normal distribution, about **68%** of all data points lie within **±1 standard deviation** of the mean. This is a well-established statistical principle known as the empirical rule or 68-95-99.7 rule. This concept is widely applied in preventive medicine for interpreting health screening results, assessing risk, and understanding disease prevalence.

**Why Each Wrong Option is Incorrect**
Option A: 50% is incorrect because only half the data lies below or above the mean, not within one standard deviation.
Option C: 95% is incorrect as it refers to values within **±2 standard deviations**, not one.
Option D: 100% is incorrect because no data point lies outside the distribution; all values are within a finite range, and 100% within one standard deviation is not statistically valid.

**Clinical Pearl / High-Yield Fact**
In preventive medicine, understanding normal distribution helps interpret screening test results—e.g., a patient's value within ±1 SD of the mean is considered normal, while values beyond ±2 SD may indicate a potential health concern.

✓ Correct Answer: B. 68%