**Core Concept**
A bilateral sagittal split osteotomy (BSSO) is a surgical procedure used to correct mandibular (lower jaw) deformities, such as prognathism (protruding jaw). The surgery involves splitting the lower jawbone on both sides to move it into a new position. The goal is to achieve optimal alignment and function of the lower jaw, improving facial aesthetics and oral function.

**Why the Correct Answer is Right**
The correct answer is determined by the limitations of the BSSO procedure. During a BSSO, the mandible is split and moved in a sagittal (front-to-back) plane. However, the extent of movement is limited by the attachment of the temporalis and medial pterygoid muscles, as well as the position of the mandibular canal. The procedure can move the mandible anteriorly (forward), posteriorly (backward), or upward, but significant downward movement is not feasible due to the attachment of the digastric muscle and the limitations of the osteotomy site.

**Why Each Wrong Option is Incorrect**
**Option A:** While a BSSO can move the mandible anteriorly or posteriorly, significant upward movement is also not feasible due to the attachment of the temporalis and medial pterygoid muscles.

**Option B:** This option is incorrect because a BSSO can move the mandible downward to some extent, but significant downward movement is not possible.

**Option C:** This option is incorrect because a BSSO can move the mandible upward to some extent, but significant upward movement is not feasible due to the attachment of the temporalis and medial pterygoid muscles.

**Clinical Pearl / High-Yield Fact**
A key limitation of the BSSO procedure is the risk of damage to the inferior alveolar nerve, which runs through the mandibular canal. This nerve supplies sensation to the lower lip and chin, and damage can result in numbness or altered sensation in these areas.

**Correct Answer: B.**