**Core Concept**
Achondroplasia is the most common cause of short-limbed dwarfism, resulting from mutations in the FGFR3 gene that encodes a fibroblast growth factor receptor. This mutation disrupts bone growth and development, leading to characteristic physical features.

**Why the Correct Answer is Right**
Achondroplasia is characterized by short stature, macrocephaly (large head size), midface hypoplasia, and a prominent forehead. The condition also often involves lumbar stenosis, foramen magnum stenosis, and hydrocephalus. These features arise due to the FGFR3 gene mutation's impact on bone growth and development.

**Why Each Wrong Option is Incorrect**
**Option A:** While not a primary feature, some individuals with achondroplasia may experience hearing loss, but it is not a defining characteristic.

**Option B:** This option is incorrect as macrocephaly is a common feature of achondroplasia, not microcephaly.

**Option C:** This option is incorrect as a prominent forehead is a characteristic feature of achondroplasia, not a flat forehead.

**Clinical Pearl / High-Yield Fact**
Achondroplasia is an autosomal dominant disorder, meaning that a single copy of the mutated FGFR3 gene is sufficient to cause the condition. Understanding the inheritance pattern is crucial for genetic counseling and prenatal diagnosis.

**Correct Answer: B. Microcephaly. Microcephaly is not a defining feature of achondroplasia.**