Deep-vein thrombosis

A deep vein thrombosis describes a large clot in one of the veins that runs deep inside the body, as opposed to those which sit just beneath the skin.  They are caused by a combination of decreased blood flow in the vein, damage to the inside of the blood vessel, and increased tendency of the blood to clot.  Their immediate cause can be trauma, infection, cancer, inflammation, stroke, heart failure, surgery, hospitalization, and long periods of immobility.  They are often found in long-haul truck drivers and passengers in economy seats on trans-Pacific flights.  Age, smoking and obesity are risk factors.
Diagnosis is by blood test (particularly D-dimers), ultrasound of the affected veins and veniogram with contrast.  An early indication is Pratt's sign - squeezing the back of the calf causes pain in the patient.
DVTs usually form in the major veins of the leg, but occasionally form in the pelvis, and very rarely in the arms.
Many DVTs are asymptomatic, and often form in hospitalized patients, generally due to their being bedridden.  However, a DVT can lead to a pulmonary embolysm if a clot breaks off and lodges in one of the pulmonary veins. 
Treatment is usually with blood thinners.  However, if the clot is large, it must often be surgically removed. 
Compression stockings or other pressurization of the leg are recommended for patients who have recently suffered from a DVT. 
In The Socratic Method, the patient suffered a DVT as the result of Vitamin K deficiency which interfered with her blood clotting factors.  The condition was initially put down to inactivity from heavy alcohol use until House noted she had none of the other signs of alcoholism.
Deep vein thrombosis at Wikipedia
Deep vein thrombosis at Mayo Clinic
Deep-vein thrombosis