The human stomach secretes an enzyme known as pepsin. this enzyme breaks down proteins into smaller chemical molecules called peptides. if pepsin was mixed in a laboratory with sugar molecules like glucose, what is the most likely result?

The question here is, is glucose a protein? No, it's not; it's a carbohydrate.

So pepsin would not break it down: it would most probably not react with glucose at all, since pepsin breaks down proteins, not carbohydrates.

Answer;
The most likely result is that Pepsin would probably NOT break down proteins in the test tube because pepsin is designed to work at very acidic pH ranges. 

Explanation;
Enzymes are very specific in that they work on a specific substrate and specific conditions such as optimum pH and temperature. Each enzyme has its own optimum pH and temperature; without which they would be inactive or denatured. For example higher temperatures higher than the optimum pH denatures the enzyme while low temperatures inactivates the enzyme.
Pepsin work best at low pH or acidic pH in the stomach, therefore, changing its pH will render it function-less and thus it wont break down proteins.