A or The?

........ disease that causes the most deaths worldwide each year is heart disease.

Which one should it be a or the?

I have chosen a ( because at the beginning of the sentence we don't know anything, which disease but my teacher said to me it is absolutely the

and his opinion is that sentence contains the most therefore it must be the) What do you think about it?

Thanks in advance.

Suppose you imagine three friends of yours.  Suppose one (A) has $100.  Another (B) has $150.  The third (C) has $200.

If you say:

A person that has the most money is C.

it means something like

An example of a person that has the most money is named C.

This implies that there may be other examples to be found of a person that has the most money.
But only one can have the most -- one particular person, so there cannot be other examples of a person with the most money.  It's contradictory.

So you have to say

The person that has the most money is C.

The same logic applies to the sentence you were asking about.

CJ

WHICH disease [that] causes the most deaths worldwide each year [is heart disease]?

When you can reformulate the phrase in the interrogative using WHICH, as in the above, you know you need the THE, because it indicates specificity (WHICH specific disease).