**Core Concept**
Merkel discs are slowly adapting mechanoreceptors found in the skin that are responsible for detecting sustained pressure and texture. They are located in the epidermis, the outermost layer of the skin, and are very sensitive to perpendicular indentation.

**Why the Correct Answer is Right**
Merkel discs are unique in that they are slowly adapting, meaning they continue to fire action potentials as long as the stimulus is applied. This allows them to provide a steady stream of information about the texture and pressure of an object. They are activated by perpendicular indentation, which means they are sensitive to pressure applied directly to the skin surface. This is in contrast to other mechanoreceptors, such as Pacinian corpuscles, which are sensitive to vibration and rapid changes in pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** Pacinian corpuscles are rapidly adapting mechanoreceptors that are sensitive to vibration and rapid changes in pressure, but not to sustained pressure or texture.

**Option B:** Meissner's corpuscles are rapidly adapting mechanoreceptors that are sensitive to light touch and changes in texture, but not to sustained pressure.

**Option D:** Ruffini end organs are slowly adapting mechanoreceptors that are sensitive to stretch and skin stretch, but not to perpendicular indentation.

**Clinical Pearl / High-Yield Fact**
It's worth noting that Merkel discs are also responsible for providing information about shape and size, and are thought to play a role in the perception of texture and form.

**✓ Correct Answer: C. Merkel discs**