**Core Concept**
The posterior mediastinum is a compartment of the mediastinum, which is the central part of the chest cavity. It is divided into three main regions: anterior, middle, and posterior. The posterior mediastinum contains several important structures, including the esophagus, trachea, and nerves, as well as various tumors, both benign and malignant.

**Why the Correct Answer is Right**
The most common posterior mediastinal tumor is the neuroblastoma, a type of cancer that arises from the neural crest cells. Neuroblastoma typically affects children and is often diagnosed in infants. It originates from the sympathetic nervous system and can metastasize to various parts of the body, including the skull, bones, and lymph nodes. The posterior mediastinum is a common site for neuroblastoma due to the presence of the sympathetic nerves.

**Why Each Wrong Option is Incorrect**
**Option A:** Lymphoma - While lymphoma can occur in the mediastinum, it is not the most common tumor in the posterior mediastinum. Lymphoma is a type of cancer that affects the immune system and can arise in various parts of the body.
**Option B:** Thymoma - Thymoma is a type of tumor that arises from the epithelial cells of the thymus, which is located in the anterior mediastinum. It is not typically associated with the posterior mediastinum.
**Option C:** Esophageal cancer - Esophageal cancer can occur in the posterior mediastinum, but it is not the most common tumor in this region. Esophageal cancer typically arises from the epithelial cells of the esophagus.

**Clinical Pearl / High-Yield Fact**
Neuroblastoma is a highly variable tumor, and its behavior can range from benign to highly malignant. It is essential to consider neuroblastoma in the differential diagnosis of posterior mediastinal tumors, especially in children and infants.

**Correct Answer:** C. Neuroblastoma