Womens PhD® Outdoor Ultra Light Micro Medium Gray - Medium. When you've got miles to cover but want the minimum between your foot and boot, the PhD® Outdoor Ultra Light Micro is ready to go. Whether it's a trail run at dawn or a hike in the heat of summer, this multi-sport sock features the power of Merino for maximum breathability, moisture management and temperature regulation. Made with Indestructawool⢠technology for ultimate durability and the 4 Degree® elite fit system to keep the sock in place.