There are two large cutaneous veins running up the arm. Both veins take origin from the dorsal venous arch of the hand and run up the lateral and medial sides of the arm. On the medial side (near the 5th digit) there is the basilic vein. On the lateral side (by the thumb), there is the cephalic vein. Since the infection is on the thumb side of the forearm, the correct answer is the cephalic vein. The median antebrachial vein runs down the center of the anterior forearm. The median cubital vein connects the cephalic vein to the basilic vein in the cubital fossa.