**Core Concept**
The medullary cavity is a hollow space within the shaft of long bones, filled with bone marrow and responsible for hematopoiesis and fat storage. In some bones, the medullary cavity is absent or greatly reduced, leading to a different bone structure.

**Why the Correct Answer is Right**
The bone that lacks a medullary cavity is the **flat bone**. Flat bones, such as the sternum, scapula, and some bones in the skull, are characterized by a dense, compact structure with no medullary cavity. This is due to the fact that these bones are designed for protection and movement rather than for housing bone marrow or storing fat.

**Why Each Wrong Option is Incorrect**
**Option A:** Long bones, such as the femur and humerus, typically have a well-developed medullary cavity. Therefore, this is an incorrect choice.
**Option B:** Short bones, like the carpals and tarsals, have a solid, compact structure but may have a small medullary cavity. This option is also incorrect.
**Option C:** Sesamoid bones, which are embedded within tendons, do not have a medullary cavity. However, the question is asking about a type of bone that has no medullary cavity, and sesamoid bones are a specific category.

**Clinical Pearl / High-Yield Fact**
When examining a patient's X-rays or CT scans, remember that the absence of a medullary cavity can indicate a bone that is not a long bone or a typical short bone. This can be an important clue in diagnosing various bone disorders or injuries.

**Correct Answer: D. Flat bone**