**Core Concept**
Corpus callosum degeneration is a condition characterized by the thinning or atrophy of the corpus callosum, which is the largest white matter structure in the brain. This degeneration can lead to various neurological symptoms and is often associated with aging, traumatic brain injury, or certain neurological disorders.

**Why the Correct Answer is Right**
The main distinctive sign of corpus callosum degeneration on ultrasound (USG) is the presence of a **hyperechogenic band**. This is due to the reduced thickness of the corpus callosum, which appears as a bright or hyperechogenic line on ultrasound images. This finding is particularly evident in the midbody of the corpus callosum. The hyperechogenic band is a result of the increased echogenicity of the remaining white matter fibers, which are more easily visualized on ultrasound.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a distinctive sign of corpus callosum degeneration on USG. Corpus callosum atrophy may lead to a reduction in the size of the corpus callosum, but this is not a specific or distinctive feature on USG.
**Option B:** This option is incorrect because it is not a characteristic finding of corpus callosum degeneration on USG. Increased echogenicity may be seen in various conditions, but it is not a specific sign of corpus callosum degeneration.
**Option C:** This option is incorrect because it is not a feature of corpus callosum degeneration on USG. Corpus callosum cysts are a separate entity and are not associated with corpus callosum degeneration.

**Clinical Pearl / High-Yield Fact**
The hyperechogenic band in corpus callosum degeneration on USG is a classic finding that should be remembered by radiologists and clinicians alike. This distinctive sign can help in the early detection and diagnosis of corpus callosum degeneration, which may be associated with various neurological disorders.

**Correct Answer:** C. Hyperechogenic band