The "4 T's" that describe the most common causes of anterior mediastinal masses are:

1) Thymoma/Thymic carcinoma

2) Teratoma (germ cell tumors)

3) Thyroid mass (goiter, cancer, nodule, etc...)

4) "Terrible" Lymphoma

The only answer choice that is not one of the classic "4 T's" is tamponade of the heart. Cardiac tamponade describes when fluid accumulates in the potential space between the heart and the pericardium, compressing the myocardium. Tamponade does not describe a solitary mass, but rather describes fluid. Furthermore, the heart is located in the middle mediastinum, not the anterior mediastinum. The anterior mediastinum's posterior border is the pericardium.

The only structure listed that is secondarily retroperitoneal is the descending colon. All of the other structures listed (kidneys, ureters, abdominal aorta, thoracic portion of esophagus) are primarily retroperitoneal, not secondarily.

A secondarily retroperitoneal structure is a structure that originally was suspended by a mesentery, but through the body's developmental phases traveled behind the peritoneum. Other secondarily retroperitoneal structures include the pancreas (except the tail), the duodenum (except the proximal aspect of the first segment), and the ascending and descending colon.

A primarily retroperitoneal structure is a structure that has been retroperitoneal throughout all of the organism's developmental phases.

Papillary muscles are located in the heart's ventricles and attach to the cusps of atrioventricular valves (mitral and tricuspid) via chordae tendineae (think of this as the tendon of the papillary muscle). Papillary muscles contract during systole (ventricular contraction) to prevent prolapse of these valves and to prevent the back flow of blood. 

Skin is the largest organ of the body. It consists of the superficial epidermis, a deeper dermis, and an even deeper fatty layer called the hypodermis. The skin has protective and sensory functions and is important in body temperature regulation, absorption, and production of vitamin D.

In human adults the spinal cord (caudal tip) usually ends between the L1 and L2 vertebrae. As there is anatomical variation the spinal cord can end between T12 and L3 and still be considered normal, however the spinal cord typically ends between the L1 and L2 vertebrae.

In newborns the caudal tip is usually found at the level of L3. 

Holocrine glands are exocrine glands that involve the rupture of the plasma membrane. In doing so, it involves the destruction of the cell. Apocrine glands only involve the shedding of the cytoplasm.

B-lymphocytes mature in the bone marrow. They can also be created and matured in the liver. Both types of lymphocyte originate from stem cells in the bone marrow, but differ in their site of maturation.

T-lymphocytes mature in the thymus, where they interact with "self antigens." This process ensures that T-lymphocytes do not interact with antigens found in the body. If the immature cell interacts with self antigens, the T-lymphocyte will be destroyed. Mature T-lymphocytes that still react to self antigens can cause allergic reactions and autoimmune disease.

Mature lymphocytes are most commonly found in the lymph nodes, where they screen the plasma and fluids for antigens.