What is the minimum amount of Sharia knowledge indispensable for a medical student, which, if acquired, would fulfill the obligation of seeking knowledge and prevent him from being negligent in his religion?

Seeking sacred knowledge is divided into two categories: an individual obligation (fardh 'ayn) and a communal obligation (fardh kifayah). The individual obligation is knowing the fundamentals of faith, the rulings of obligatory acts of worship like prayer and zakat (definition), discerning the definitively lawful and unlawful (that which is known by necessity to be part of the religion), and the rulings of transactions that a Muslim engages in. This much is easy and can be acquired with ease. As for the communal obligation, it is whatever exceeds that. If a sufficient number of people undertake it, the sin is lifted from the rest. And it is befitting for one who has the ability to increase in sacred knowledge, for it is among the greatest acts of drawing near to Allah.