**Core Concept**
Cowden's syndrome is a rare genetic disorder characterized by multiple hamartomas (benign growths) in various organs, including the skin, gastrointestinal tract, breast, thyroid gland, and genitourinary system. This condition is caused by mutations in the PTEN gene, leading to an increased risk of cancer development.

**Why the Correct Answer is Right**
Cowden's syndrome is distinguished by the presence of multiple hamartomatous tumors in the skin (oro-cutaneous), gastrointestinal tract (gastrointestinal polyps), breast, thyroid gland, and genitourinary system. The PTEN gene, a tumor suppressor gene, plays a crucial role in regulating cell growth and division. Mutations in PTEN lead to uncontrolled cell growth, resulting in the development of hamartomas and an increased risk of cancer. The combination of these clinical features is pathognomonic for Cowden's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Familial adenomatous polyposis (FAP) is characterized by the development of numerous adenomatous polyps in the colon, leading to an increased risk of colorectal cancer. While FAP can involve extracolonic manifestations, such as osteomas and desmoid tumors, the specific combination of features described in the question is not typical of FAP.
**Option C:** Peutz Jeghers' syndrome is a genetic disorder characterized by the development of hamartomatous polyps in the gastrointestinal tract and an increased risk of various cancers. However, it is distinguished by the presence of melanin deposits on the skin and mucous membranes, which are not mentioned in the question.
**Option D:** Juvenile polyposis is a rare genetic disorder characterized by the development of hamartomatous polyps in the gastrointestinal tract, but it is not associated with the same range of extracolonic manifestations as Cowden's syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Cowden's syndrome is associated with a significantly increased risk of various cancers, including breast, thyroid, and endometrial cancer. Patients with Cowden's syndrome should undergo regular screening and surveillance for these malignancies.

**✓ Correct Answer: B. Cowden's syndrome**