**Core Concept**
Two-point discrimination is a measure of the minimum distance between two points that can be perceived as separate by the human sense of touch. This distance varies across different body regions, with the fingertips being the most sensitive. The ability to perceive two points as separate is mediated by mechanoreceptors in the skin, which are sensitive to changes in pressure and texture.

**Why the Correct Answer is Right**
The fingertips contain a high concentration of Meissner's corpuscles, a type of mechanoreceptor that is responsible for detecting light touch and changes in texture. These receptors are particularly sensitive in the fingertips due to the thinness of the skin and the presence of nerve endings. When two points are brought close together on the fingertips, the Meissner's corpuscles can detect the subtle changes in pressure and texture, allowing the individual to perceive the two points as separate. The minimum distance for two-point discrimination on the fingertips is typically around 2-3 mm.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low. Two-point discrimination thresholds are generally higher than 1 mm.
**Option B:** This option is incorrect because it is too high. While two-point discrimination thresholds can vary across different body regions, the fingertips are generally one of the most sensitive areas.
**Option C:** This option is incorrect because it is not a specific enough value. While two-point discrimination thresholds can vary, a specific value like 4 mm is not typically considered a standard threshold.

**Clinical Pearl / High-Yield Fact**
It's worth noting that two-point discrimination thresholds can be affected by various factors, including age, sex, and neurological conditions. For example, individuals with peripheral neuropathy may have impaired two-point discrimination due to damage to the nerve endings in the skin.

**Correct Answer: D. 2-3 mm**