One of them is not a true cyst:
**Core Concept**
A cyst is a closed capsule or sac-like structure that contains fluid, semisolid, or gaseous material. In medical terminology, a cyst can be distinguished from other types of lesions based on its characteristics, such as being a true or false cyst. True cysts are lined by epithelial cells, whereas false cysts are not.
**Why the Correct Answer is Right**
A true cyst, such as a dermoid cyst or a mucinous cystadenoma, is lined by epithelial cells. This lining is responsible for the formation and maintenance of the cystic structure. For example, in a dermoid cyst, the lining may produce keratin, causing the cyst to grow and expand. In contrast, a false cyst, such as a pseudocyst, does not have an epithelial lining and is typically filled with fluid or semi-solid material.
**Why Each Wrong Option is Incorrect**
**Option A:** A hydatid cyst, caused by Echinococcus granulosus, is a true cyst because it is lined by an epithelial layer called the germinal layer.
**Option B:** A ganglion cyst, also known as a synovial cyst, is a true cyst that arises from the synovial tissue of joints and is lined by a fibrous capsule.
**Option C:** A pseudocyst, unlike a true cyst, lacks an epithelial lining and is typically filled with fluid or semi-solid material.
**Clinical Pearl / High-Yield Fact**
When evaluating a cystic lesion, it is essential to determine whether it is a true or false cyst, as this information can guide treatment decisions and help predict the likelihood of recurrence.
**Correct Answer: D. Pseudocyst. Pseudocyst.**