## Core Concept
A retrosternal goiter, also known as a substernal goiter, is an enlargement of the thyroid gland that extends below the sternum. It can be an extension of a cervical goiter or a primary substernal thyroid enlargement. The substernal space is a potential space in the thoracic cavity.

## Why the Correct Answer is Right
The correct answer involves understanding the anatomical location and clinical presentation of a retrosternal goiter. These goiters can cause compressive symptoms such as dysphagia, dyspnea, and can sometimes be associated with superior vena cava syndrome. The surgical approach often involves a combination of cervical and sometimes sternotomy approaches for large goiters.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe the typical presentation or management of a retrosternal goiter.
- **Option B:** This option is incorrect as it may not specifically relate to the characteristics or treatment modalities of retrosternal goiters.
- **Option D:** This option is incorrect because it does not provide a correct or relevant description of retrosternal goiter.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that retrosternal goiters can often be diagnosed with imaging studies such as CT or MRI of the chest, which help in planning the surgical approach. These goiters can sometimes be asymptomatic and discovered incidentally on imaging done for other reasons.

## Correct Answer Line
**Correct Answer: C.**