Which one of the following veins my have valves only 25% of the time?

The external iliac vein is known to have valves present only about 25% of the time. This low prevalence is significant in understanding the venous anatomy and hemodynamics in this region of the body. Valves in veins serve the purpose of preventing retrograde flow, particularly in lower extremities where blood must travel against gravity to return to the heart. However, in certain larger veins, such as the external iliac, the low incidence of valves reflects their relatively high capacity and the fact that they already possess significant support from neighboring structures in the pelvis.

In contrast, other veins listed, such as the inferior vena cava, often contain no valves at all as it carries a large volume of blood from the lower body directly to the right atrium and does not require the assistance of valves. The subclavian vein also typically does not contain valves, as its anatomy and path allow for efficient blood return from the upper extremities. The internal jugular vein might have valves, but they are more common in the upper segments and serve to aid in preventing the backflow of blood from the heart during activities like respiration.

Understanding the anatomical variations and functions of these veins is crucial for interpreting venous flow dynamics, which can have clinical implications when performing ultrasound