And they did it, aptly enough, on Father's Day.
"He downplayed how hard it was. He was like, 'Oh, yeah, you can do this,' " said Lauren Fox, a senior at the Agnes Irwin School in Rosemont.
He was right, as fathers often are, but the two-day climb over glaciers on narrow pathways with death-defying drops was the toughest thing she had ever done, said the champion swimmer who has enough ribbons to fill a wall in her Penn Valley home.
"I didn't want to discourage her," said Bill Fox, 71, the adventurer-in-chief of a family of three thrill-seeking sons and a daughter. Besides mountain climbing, he and his sons love to race cars.
Fox became a public health officer in the Peace Corps to fulfill his military duty during the Vietnam War. He volunteered for Bolivia because it had among the poorest people and the highest mountains in the Western Hemisphere.
"We grew up hearing his stories about the mountain," said Lauren Fox, who is interning in the diabetes lab at the Hospital at the University of Pennsylvania this summer.
It's difficult to explain the allure of mountains to those who do not climb. Climbing is grueling, and requires skill and close attention to weather. And it is impossible to remove the element of risk even on a smaller mountain like Huayna Potosi. (In comparison, Everest is 29,000 feet and requires bottled oxygen.)
Lauren's mother, Laurie Kilpatrick, gets altitude sickness, but all four of Bill Fox's children have traveled with him on his yearly trips to Bolivia, where he teaches in a medical school and the family volunteers in an Amazon village, helping fund projects such as a bread business and orphanage. Lauren made her first trip when she was 11, and has been back six times.
Last summer, she and half-brother James Fox, a professional car racer who lives in California, got the mountain-climbing bug. After all, her two oldest half-brothers, ages 41 and 44, had conquered Bolivian mountains, and one climbed Mount Kilimanjaro with their father, so they figured it was time to get out the ice ax.
To prepare for a tough climb, Lauren trained with a local climber, borrowed equipment, and stocked up on freeze-dried food. Her father, who no longer climbs, hired a guide he has known for 20 years but who only speaks Spanish. Fortunately, Lauren does as well.
"They're all levelheaded and had strict instructions that if there's any question, turn around and go back," said Bill Fox, who climbed the same mountain in 1969 in jeans, an Army jacket, and old leather boots. He knew of no guides then, so he and his friends asked locals if they knew the way up and "they roughly aimed you," he recalled.
In early June, Lauren and James went to La Paz and spent a week acclimating to being 13,000 feet above sea level, running up and down six flights of stairs in their hotel with a backpack.
On June 15, they set out for the base camp and practiced walking with boots and crampons and climbing an ice wall. Instead of staying in a small brick refuge used by other climbers, they opted for their dad's old tent, which Lauren felt was "more authentic."
The second day, they awoke at 8 a.m. and started climbing. With porters carrying most of their gear, they hiked for three to four hours and set up camp at 18,000 feet.
They began their final ascent the next day at 3 a.m., guided by headlamps and following the carefully placed footsteps of their guide.
"You can't see, you can't breathe, you're carrying food and water because the porters don't come with you. At one point the water in our tubes froze, so we got a little dehydrated," recalled Lauren.
Near the top was a narrow ridge - "Scary as hell," her father remembered - about six inches wide with a 3,000-foot drop on one side and a 1,500-foot drop on the other.
"I stayed very focused on the people's feet in front of me," she said, but in a picture she is beaming as she balances above the clouds and the tips of lesser peaks. She looks, quite literally, on top of the world.
From that shot, it's easy to understand the power of mountains to thrill. Although when her mother saw it, she thought, "I'm glad I didn't see the pictures before she went," said Kilpatrick, an associate professor at Temple Medical School.
When the climbers finally reached the summit, they were the only ones there.
"It's so peaceful. The view is amazing. . . . We were trying to take it all in, but I've never been so exhausted in my life," Lauren said.
And ahead of them was the return trip, made torturous by boots that were too big. "My toes kept smashing against the front. I was kind of on the verge of tears," she said.
Waiting for them back at the hotel was her proud father.
"She's a tough lady. I love that," he said.
How tough? Next year, she and her brother want to climb Illimani, the "Beast of Bolivia," higher than Huayna Potosi by 1,000 feet and more dangerous - people have died trying to reach the top.
But don't expect Bill Fox to discourage her.
"It's extremely safe these days," he said.
Contact Kathy Boccella at 610-313-8123, firstname.lastname@example.org or @kmboccella on Twitter.