In general, where do malignancies tend to arise in an organ?

Malignancies typically arise in the parenchyma of an organ. The parenchyma refers to the functional tissue of an organ as opposed to the supporting tissue, which is known as the stroma. In most organs, it is the parenchymal cells that carry out the primary functions of that organ, and when these cells undergo malignant transformation, it leads to the development of cancer.

For example, in organs like the lungs or liver, the cancer originates from the epithelial cells (lining cells) or other types of parenchymal cells that perform the major tasks of that organ. As a result, those malignant cells proliferate and form tumors within the parenchymal area, leading to the dysfunction of the organ and the manifestation of cancer-related symptoms.

Other areas like the stroma or connective tissue are often involved in supporting the parenchyma, but they are not typically the primary sites where malignant tumors originate. The bloodstream is not a site for primary malignancy but can be involved in the spread (metastasis) of cancer from one location to another. Thus, the parenchyma is the correct choice as it is the primary site for the development of malignancies within an organ.