On a T2-weighted MRI of the brain, which structure appears as a distinct hypointense (dark) line separating the caudate nucleus and thalamus from the putamen and globus pallidus?

• A External capsule
• B Internal capsule (posterior limb) ✓
• C Extreme capsule
• D Claustrum

Explanation

The internal capsule is composed of densely packed white matter (myelinated axons) and appears hypointense on T2-weighted MRI due to its high myelin content. It separates the caudate nucleus and thalamus (medially) from the lentiform nucleus (putamen and globus pallidus, laterally). The posterior limb of the internal capsule contains the corticospinal tract, and infarction here (from lenticulostriate artery occlusion) causes dense contralateral hemiplegia. The external capsule lies between the putamen and claustrum; the extreme capsule lies between the claustrum and the insular cortex.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.