**Core Concept**
The Recommended Dietary Allowance (RDA) is a daily intake level of a nutrient that meets the needs of 97-98% of healthy individuals. To calculate the RDA, the Estimated Average Requirement (EAR) is increased by 2 standard deviations (SD) to account for the needs of individuals who require more of the nutrient. This is based on the concept that the distribution of nutrient requirements is approximately normal.

**Why the Correct Answer is Right**
The RDA is not increased by 2 SD for a nutrient when the EAR is based on a single measurement or limited data. In such cases, the RDA is set equal to the EAR, and no additional increase is made. This is because the EAR is derived from a single value, and adding 2 SD would not accurately reflect the needs of the population.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll start with **Option B:** and **Option C:**.
**Option B:** Incorrect because this option is not related to the calculation of RDA.
**Option C:** Incorrect because this option is not related to the calculation of RDA.

**Clinical Pearl / High-Yield Fact**
When calculating RDA, it's essential to note that the EAR is the starting point, and adding 2 SD is only done when sufficient data are available to support this increase. This nuance is crucial in accurately determining the RDA for a particular nutrient.

**Correct Answer:** D.