Which of the following reactions would suggest a GNR is urease positive?

A Gram-negative rod (GNR) that is urease positive will produce the enzyme urease, which hydrolyzes urea into ammonia and carbon dioxide. The formation of ammonia increases the pH of the medium, resulting in a color change due to a pH indicator present in the medium.

In this context, a color change to pink indicates a rise in pH, confirming the presence of ammonia as a byproduct of the urease activity. Therefore, when the media changes to pink, it suggests that the GNR is capable of metabolizing urea, demonstrating urease positivity.

The other reactions provided do not indicate urease activity. No color change in the media would suggest that there was no urease activity or that other metabolic processes did not involve urea. The formation of gas bubbles could indicate fermentation or other reactions but does not specifically correlate with urease activity. A yellow color in the media typically indicates acidity, which is not consistent with urease activity that results in alkalinization. Thus, the pink color change is a definitive marker of urease positive strains.