Failure in a particular structure of the heart tends to cause a backup of blood in the lungs, known as pulmonary congestive heart failure. Failure of which structure of the heart would lead to such a backup?

Left ventricle.

Explanation:

Human heart is divided into four chambers - two auricles and two ventricles. Heart acts as the pumping organ and carries both oxygenated and deoxygenated blood in the right and left ventricles.

The failure of functioning of any chamber of heart may cause a particular disease in the individuals. The pulmonary congestive heart failure occurs due to the failure of the left ventricles of the heart. The left ventricle carries blood from heart to the body tissues, its failure leads to the backup of blood in the lungs.

Thus, the answer is left ventricle.