The tunica externa is typically the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.

A. Skeletal: Precapillary sphincters are not composed of skeletal muscle. Skeletal muscle is voluntary and striated, controlling movement rather than regulating microcirculatory blood flow.

B. Circular smooth: Precapillary sphincters are made of circular smooth muscle cells that encircle the entrance to capillary beds. Their contraction and relaxation regulate blood flow into individual capillaries, allowing tissues to control perfusion according to metabolic demand.

C. Cardiac: Cardiac muscle is found only in the heart and is specialized for rhythmic contraction. It does not exist in the walls of precapillary sphincters and therefore cannot mediate local control of capillary perfusion.

D. Involuntary: While precapillary sphincters are involuntary in function because they are smooth muscle, the more precise description of their structure is circular smooth muscle. “Involuntary” describes functional control rather than anatomical composition.

A. Small saphenous vein → Posterior tibial vein → Fibular vein → Femoral vein → Inferior vena cava: The small saphenous vein drains into the popliteal vein, not directly into the posterior tibial or fibular veins. Additionally, deep veins such as the posterior tibial and fibular veins merge before forming the popliteal vein, making this sequence anatomically incorrect.

B. Great saphenous vein → Femoral vein → External iliac vein → Internal iliac vein → Inferior vena cava: The great saphenous vein drains the medial side of the foot and leg, not the lower lateral side. Furthermore, the external and internal iliac veins join to form the common iliac vein, so the sequence listed is out of proper anatomical order.

C. Small saphenous vein → Popliteal vein → Femoral vein → External iliac vein → Common iliac vein → Inferior vena cava: The small saphenous vein drains superficial blood from the lateral side of the foot and posterior leg into the popliteal vein. The popliteal vein becomes the femoral vein, which continues as the external iliac vein, then joins the internal iliac to form the common iliac vein, ultimately draining into the inferior vena cava.

D. Popliteal vein → Femoral vein → Great saphenous vein → Common iliac vein → Inferior vena cava: The great saphenous vein drains into the femoral vein rather than receiving blood from it. This sequence reverses the direction of normal venous flow and incorrectly places a superficial vein after deep venous structures.

E. Posterior tibial vein → Fibular vein → External iliac vein → Inferior vena cava: The posterior tibial and fibular veins are deep veins and do not directly receive superficial blood from the lower lateral foot. Additionally, the pathway skips the popliteal, femoral, and common iliac veins.