I am under the impression that most pronouns in Polish are capitalized; therefore, I would have written "Twój" instead of "twój".... can someone please clarify this for me? Thanks in advance!
You only do that when writing a letter to someone as a sign of respect - it's just a polite thing to do.
Otherwise, lower case letter is fine.
You can spell it with a capital to show respect towards the person addressed (and you typically should do it in letters), but it's not required normally.
Why is "Is your uncle a physician" not accepted ?
Oversight, added "physician".