**Question:** Which of the following parts of corpus callosum develops first -

A. Splenium
B. Genu
C. Ipsilateral parts (body and genu)
D. Contralateral parts (body and genu)

**Core Concept:** The corpus callosum is a bundle of nerve fibers that connects the two cerebral hemispheres of the brain. It consists of three main parts: the genu, body, and splenium. During brain development, the corpus callosum undergoes a specific sequence of events, which is crucial for understanding its formation and maturation.

**Why the Correct Answer is Right:** The correct answer is D, "Contralateral parts (body and genu) develop first." In the early stages of brain development, the contralateral parts, specifically the body and genu, start to form first. This sequence is essential for the proper integration of information between both hemispheres of the brain, allowing for efficient communication and coordination between the cerebral cortexes.

**Why Each Wrong Option is Incorrect:**

A. The splenium develops last, not first. It is the posterior part of the corpus callosum and forms later on in development.

B. The genu also develops last, forming after the contralateral parts.

C. The ipsilateral parts (body and genu) develop last, not first, as they form later during brain development.

**Why the Contralateral Parts Develop First:**

The contralateral parts, specifically the genu and body of the corpus callosum, develop first because they need to establish connections between the homologous areas in the two hemispheres. This allows for the integration of sensory, motor, and cognitive information early in development, enabling the brain to function optimally.

**Clinical Pearl:** Understanding the developmental sequence of the corpus callosum is crucial for understanding brain function and potential issues that may arise if it is not formed correctly. Defects in corpus callosum development can lead to conditions like corpus callosum agenesis, where the structure does not form at all, or corpus callosum hypoplasia, where the structure is underdeveloped. These conditions can result in neurological impairments, such as intellectual disability, developmental delays, and social communication issues.

**Correct Answer:** D. Contralateral parts (body and genu) develop first.