**Core Concept**
The design of choice for endodontic microsurgery involves creating an adequate surgical access to the root-end area while maintaining the integrity of the periodontal tissues. In endodontic microsurgery, a full-thickness flap is preferred to ensure proper visualization and access to the surgical site.

**Why the Correct Answer is Right**
A sulcular full-thickness flap is the preferred design in endodontic microsurgery due to its ability to provide optimal access to the root-end area while preserving the periodontal tissues. This design involves incising the mucoperiosteal flap through the sulcus, allowing for a wide exposure of the surgical site. The full-thickness nature of the flap enables the surgeon to visualize and access the root-end area more effectively, facilitating precise and accurate surgical procedures.

**Why Each Wrong Option is Incorrect**
**Option A:** The sulcular flap is more commonly used in periodontal surgery and does not provide the same level of access as a full-thickness flap in endodontic microsurgery.

**Option B:** A sulcular partial-thickness flap is not suitable for endodontic microsurgery as it may compromise the integrity of the periodontal tissues and limit access to the surgical site.

**Option D:** The envelope flap is a type of flap used in oral and maxillofacial surgery, but it is not the design of choice for endodontic microsurgery due to its limited access to the root-end area.

**Clinical Pearl / High-Yield Fact**
In endodontic microsurgery, the design of the flap should be tailored to the specific needs of the case, with the goal of achieving optimal access while preserving the periodontal tissues. A sulcular full-thickness flap is generally preferred due to its ability to provide a wide exposure of the surgical site.

**✓ Correct Answer: C. Sulcular full-thickness flap.**