Can a doctor who is content with knowing what is necessary for salvation and understanding the correct way to perform his acts of worship and maintain his creed attain the rank of Siddiq (the truthful one) or of the working scholars?

The rank of Siddiqiyah (truthfulness/sincerity) combines perfect devotion to Allah and submission to the Messenger, peace be upon him, and its essence is certainty (yaqeen). The Siddiqoon (sincere ones) vary in their degrees. Any professional can reach the rank of Siddiqiyah and that of the working scholars, provided they possess perfect certainty, devotion to Allah, and submission to His Messenger, peace be upon him.