**Core Concept:** Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. This leads to developmental and physiological abnormalities, including cognitive impairment and characteristic physical features.

**Why the Correct Answer is Right:** In Down's syndrome, the presence of an extra copy of chromosome 21 alters brain development, particularly affecting the size and shape of the skull. This results in a distinctive head shape, which is triangular or "seesaw" in appearance.

**Why Each Wrong Option is Incorrect:**

A. Macrocephaly (enlarged head) is not a feature of Down's syndrome. In fact, the head size is usually within normal limits or even slightly smaller.

B. Mental retardation is a common symptom in Down's syndrome, but the head shape is not typically described as "arachnoid" or "spider-like."

C. The term "hypoplastic" refers to underdevelopment or small size, which is not applicable to the head shape in Down's syndrome.

D. The correct answer of a triangular or "seesaw" head shape is the most distinctive physical feature of Down's syndrome, distinguishing it from other conditions.

**Clinical Pearl:** The characteristic "triangular" head shape in Down's syndrome is a useful clinical clue for diagnosing this condition, even in infants and young children who may not yet display other typical features like flattened nose, short neck, and increased skin pigmentation.

**Correct Answer:** D. triangular or "seesaw" head shape