**Core Concept**
Retrosternal goiter, also known as substernal goiter, is a condition where a portion of the thyroid gland extends below the sternum into the thoracic cavity. This can cause various symptoms due to compression of adjacent structures.

**Why the Correct Answer is Right**
The most common presenting feature of retrosternal goiter is dyspnea, or shortness of breath. This occurs due to compression of the trachea, which can lead to narrowing of the airway and reduced airflow. The retrosternal goiter can also compress the esophagus, leading to dysphagia (difficulty swallowing). However, dyspnea is the most common symptom due to the critical nature of airway compression.

**Why Each Wrong Option is Incorrect**
* **Option A:** While hoarseness of voice (dysphonia) can be a symptom of retrosternal goiter due to compression of the recurrent laryngeal nerve, it is not the most common presenting feature.
* **Option B:** Chest pain can be a symptom of retrosternal goiter due to compression of the mediastinal structures, but it is not as common as dyspnea.
* **Option C:** Cough, while a common symptom of many respiratory conditions, is not typically the most common presenting feature of retrosternal goiter.

**Clinical Pearl / High-Yield Fact**
A key clinical clue to retrosternal goiter is a palpable thyroid gland that extends below the clavicle. This can be confirmed on imaging studies such as chest X-ray or CT scan.

**Correct Answer: C. Dyspnea**