MRCP2-2392

A 32-year-old woman presents with dyspnoea and pleuritic chest pain. An elevated D-dimer and subsequent CTPA reveal a pulmonary embolism. She has a history of giving birth to her daughter 18 months ago (full term, vaginal delivery) and anxiety. Her 52-year-old mother has had two deep vein thromboses in the past 10 years. What is the most probable underlying cause?