Black students applying to medical school have a lower mean aptitude for medicine, as demonstrated by the MCAT. Aptitude is not the ability to perform a specific task; that is achievement. All kinds of factors influence aptitude. Take lead poisoning. It will lower aptitude. Note, it won't just lower your aptitude score: it will lower you aptitude, which is the reason you get a lower score.
I think we have to agree on a definition of the word "aptitude" before we can go any further.
Wikipedia has this: "An aptitude is a component of a competency to do a certain kind of work at a certain level, which can also be considered "talent". Aptitudes may be physical or mental. Aptitude is not knowledge, understanding, learned or acquired abilities (skills) or attitude. The innate nature of aptitude is in contrast to achievement, which represents knowledge or ability that is gained."
If I have a poor Algebra teacher I will probably achieve less than I would with a good one. That doesn't mean my innate abilities will be reduced. It means my innate abilities will be underutilized. My aptitude is what I bring to the class, not what I get out of it.
Black students have lower aptitude than White students by the end of Year 12. This is indisputable. The reasons for this difference are probably manifold; indeed people have already named group differences between Blacks and Whites on variables likely to affect aptitude e.g. lead contamination and quality of early education.
In this thread, I have talked about both achievement and aptitude. Both are correlated with academic success, and they each explain variance in academic success that the other does not (that is, although they are not independent of each other, they are still both useful together to predict academic success).
There are many aptitude tests available, ranging from measures of general mental ability to highly specialised aptitude measures, like clerical checking or medical aptitude.
Of the people who take tests of medical aptitude, Asians score the highest, followed by Whites, followed by other groups (in the U.S.). Medical aptitude tests measure medical aptitude, and higher scores represent better aptitude. If they did not, they'd be worthless and medical schools would throw them out.
But they're not worthless. Indeed, they're very, very good measures of aptitude, and that's why 95% of the people who score in the highest range get admitted to medical school.
If someone scored higher than you on an aptitude test then, within the limits of the margin of error, they probably have more aptitude than you on whatever the test was measuring. Aptitude tests are blind to the reasons for the difference: you could be genetically less endowed, you could have had lead poisoning, you could have had a poor early environment, you could have had a traumatic brain injury, you could have lowered your aptitude by substance abuse.
The reason you've got lower aptitude does not make a difference. Lower aptitude is lower aptitude, and anything you do to a person who is over 21 can't change that.