Text Explanation

Caudate Nucleus.: This answer choice is incorrect, because the caudate nucleus is a component of the basal ganglia (basal nuclei). The MCAT is not necessarily expecting you to know what the caudate nucleus is. Rather, it is expecting you to reason that the caudate nucleus is a nucleus, which means it is gray matter. Hence, it might very well be part of basal ganglia, which are also gray matter.

Substantia Nigra.: This answer choice is incorrect, because the substantia nigra is a component of the basal ganglia. The substantia nigra is gray matter that comprises a portion of the mid-brain. It is implicated in Parkinson's disease and other movement disorders. While you might not be familiar with the substantia nigra, for the MCAT, you do not need to know exactly what it is in order to answer this question correctly. This question is really testing your ability to differentiate between white and gray matter. The basal ganglia (nuclei) are gray matter, as is the substantia nigra.

Corpus Callosum.: This answer choice is correct. For the MCAT, you are not required to memorize all of the components of the basal ganglia (basal nuclei), although you will have to in medical school. To answer this question correctly, you simply have to have a decent understanding of the what the basal ganglia are and do, and then you can figure out which one is out of place.

The basal ganglia are sections of gray matter in the subcortical region of the cerebrum and the midbrain. They are involved in coordinated movement, the formation of habits, and the reward pathways of the brain.

The corpus callosum is a large collection of axons (white matter) that connects the left and right hemispheres. The corpus callosum is not a nucleus at all, because it white matter, not gray matter. Therefore, this is the correct answer.

Putamen.: This answer choice is incorrect, because the putamen is a component of the basal ganglia (basal nuclei). There is a good chance you are not familiar with the putamen, but for the MCAT, knowledge of the putamen is not required to answer this question correctly. This question is really testing you on whether you know the difference between white and gray matter in the central nervous system. Basal ganglia (nuclei) are gray matter. The putamen is also gray matter, so this answer choice can be eliminated.