Which of the following is associated with antithrombin deficiency?

Antithrombin deficiency is a condition that leads to an increased risk of thrombosis, which refers to the formation of blood clots within blood vessels. Antithrombin is a critical protein that regulates blood coagulation by inhibiting several key enzymes in the coagulation cascade, particularly thrombin and factor X. When antithrombin levels are deficient, the body lacks an essential mechanism to limit clot formation, resulting in a hypercoagulable state.

Individuals with antithrombin deficiency are, therefore, at a higher risk of developing venous thromboembolism (VTE), which includes conditions like deep vein thrombosis (DVT) and pulmonary embolism (PE). This increased tendency for clot formation is why thrombosis is the correct answer, as it directly correlates with the pathology associated with antithrombin deficiency.

In contrast, thrombocytosis, thrombocytopenia, and bleeding are not the primary concerns associated with antithrombin deficiency. Thrombocytosis refers to an elevated platelet count, which can occur for various reasons, often unrelated to antithrombin levels. Thrombocytopenia, a condition characterized by a low platelet count, can lead to bleeding issues, but it's not directly linked to