**Core Concept**
Achondroplasia is the most common cause of short-limbed dwarfism, resulting from a mutation in the FGFR3 gene. This leads to impaired endochondral ossification, causing characteristic skeletal abnormalities.

**Why the Correct Answer is Right**
Achondroplasia is characterized by short stature, macrocephaly, and characteristic facial features. Mental retardation is not a typical feature of achondroplasia, and individuals with this condition usually have normal intelligence. The condition is inherited in an autosomal dominant pattern, not autosomal recessive.

**Why Each Wrong Option is Incorrect**
**Option A:** Defective head - This is incorrect because achondroplasia is actually associated with macrocephaly, not defective head.
**Option C:** Autosomal recessive - This is incorrect because achondroplasia is inherited in an autosomal dominant pattern.
**Option D:** Familial - This is incorrect because achondroplasia is a genetic disorder that can occur sporadically or be inherited in a familial pattern.

**Clinical Pearl / High-Yield Fact**
Achondroplasia is the most common cause of short-limbed dwarfism, accounting for approximately 70% of all cases. It is essential to recognize the characteristic features of achondroplasia to avoid misdiagnosis.

**Correct Answer: D. Familial**