What is the primary characteristic of polycystic kidney disease?

Polycystic kidney disease (PKD) is fundamentally characterized by the growth of numerous cysts in the kidneys. These cysts are generally benign, meaning they are non-cancerous, but their presence leads to significant kidney enlargement and can disrupt normal kidney function over time. As the cysts increase in size, they can replace healthy kidney tissue, impairing the organ's ability to filter blood and produce urine effectively.

In contrast, malignant tumors (the first option) are associated with cancer and do not represent the nature of PKD. Inflammation of kidney tissue (the third option) refers to different kidney conditions, such as nephritis, rather than PKD. Lastly, while obstruction of urinary flow (the fourth option) can occur in various renal disorders, PKD itself is primarily about the formation of cysts leading to kidney dysfunction rather than direct obstruction. Thus, the benign cyst growth and its potential to disrupt kidney function define the primary characteristic of polycystic kidney disease.