Which of these choices is NOT a source of adult stem cells?

The correct choice refers to a blastocyst, which is an early stage of embryonic development. At this stage, the cells are considered to be pluripotent and are known as embryonic stem cells. These stem cells have the ability to differentiate into nearly any cell type in the body, which is a key characteristic of embryonic stem cells, not adult stem cells.

On the other hand, the other options—heart, fat, and umbilical cord tissue—are indeed sources of adult stem cells. Adult stem cells are typically multipotent, meaning they have a more limited differentiation potential than embryonic stem cells. For instance, stem cells found in the heart are involved in the repair and regeneration of cardiac tissue. Fat (adipose tissue) contains mesenchymal stem cells that are capable of differentiating into various cell types, including adipocytes, osteoblasts, and chondrocytes. Umbilical cord tissue is a rich source of mesenchymal stem cells as well, which can contribute to regenerative therapies.

Thus, the blastocyst is not a source of adult stem cells but rather a source of embryonic stem cells, making this the correct answer.