**Core Concept**
Hassall's corpuscles are a histopathological feature of the thymus gland, composed of concentric layers of epithelial cells that undergo apoptosis and form a characteristic structure. They play a crucial role in the maturation and selection of T-lymphocytes in the thymus. The presence of Hassall's corpuscles is a distinctive feature of the medulla of the thymus.

**Why the Correct Answer is Right**
The correct answer is linked to the role of the thymus in T-cell maturation and selection. Hassall's corpuscles are formed by the concentric layers of epithelial cells that undergo programmed cell death (apoptosis) and are a hallmark of the medulla of the thymus. This process helps to eliminate self-reactive T-cells and allows the mature T-cells to migrate to the periphery and perform their immune functions. The presence of Hassall's corpuscles is a key feature that distinguishes the thymus from other lymphoid organs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hassall's corpuscles are not a feature of the spleen, which is another lymphoid organ involved in the immune response.
**Option B:** This option is incorrect because Hassall's corpuscles are not a feature of the lymph nodes, which are sites of immune cell activation and proliferation.
**Option C:** This option is incorrect because Hassall's corpuscles are not a feature of the bone marrow, which is the site of hematopoiesis and the production of all blood cells.

**Clinical Pearl / High-Yield Fact**
The presence of Hassall's corpuscles is a key feature that distinguishes the thymus from other lymphoid organs, and their absence can be seen in conditions such as thymic aplasia or dysplasia.

**Correct Answer:** D. Thymus