"Medicine," replied the skinny 19-year-old, according to his younger sister, Maria Elena.
One morning, though, his mother got a surprise. In the room, she found not anatomy or medicine texts but books on theology and Catholicism. Perturbed at his change of course, she confronted her eldest son.
"What is this?" she asked.
Bergoglio responded calmly: "It's medicine for the soul."
The first pope from Latin America is also the first Jesuit pope. Like priests from other orders, Jesuits take vows of poverty, chastity and obedience, as well as a fourth special vow of obedience to the pope. They also make a promise to refrain from seeking high Church offices.
But Bergoglio rose steadily through the order's leadership posts and beyond, sometimes crossing swords with colleagues and once proving so meddlesome that a Jesuit boss dismissed him from the school where he was teaching. After being named a bishop he climbed through the Church hierarchy itself, rising to lead Argentina's largest archdiocese and eventually being named a cardinal.
When his name emerged as a possible successor to John Paul in 2005, Bergoglio told family, friends and Argentine media that he didn't want to be pope. He loved Buenos Aires too much, he said. He had no desire to leave.
When the conclave named him successor to Pope Benedict earlier this month, he joked: "May God forgive you."
"Jorge is a political man with a keen nose for politics," says Rafael Velasco, a Jesuit priest and former colleague who is now rector of the Catholic University of Cordoba, in central Argentina. "It's not an act, the humility. But it's part of his great capacity to intuitively know and read people."
After his ordination in 1969 and a brief assignment in Spain, Bergoglio returned to Buenos Aires to run the order's program for initiates. There, he quickly impressed superiors, according to fellow Jesuits from the period. In 1973, aged 36, Bergoglio was chosen as the order's national leader, or "provincial," a post that usually lasts six years.
But Bergoglio's tenure coincided with one of the most tumultuous periods in Argentina's history. Like much of the rest of Latin America, the country was riven by economic crisis and growing conflict between right and left. Some members of the regional Church were beginning to flirt with Liberation Theology, a movement that sought to empower the poor. Priests at the extremes of the movement began to advocate armed struggle.
Though Bergoglio had worked for the poor, he made it clear in discussions that the order would not stray too far toward Marxism, according to several of his successors as provincial as well as other Jesuit officials.