Medical Laboratory Scientist ASCP Practice Exam 2026 – Complete Study Guide

The correct choice represents species that demonstrate a lysine decarboxylase positive reaction, an arginine decarboxylase negative reaction, and an ornithine decarboxylase positive reaction. In the case of Enterobacter aerogenes, Edwardsiella species, and most Serratia species, they are capable of decarboxylating lysine and ornithine, which means they can convert these amino acids into basic amines, thereby raising the pH of the medium and resulting in a color change to alkaline. This metabolic characteristic is crucial for differentiating these organisms from others during microbial identification.

In contrast, the other listed options include species that either do not fit these biochemical profiles or have different reactions concerning lysine, arginine, and ornithine. For instance, Escherichia coli and Klebsiella pneumoniae would not exhibit the same pattern of amino acid metabolism as the first group. Similarly, Salmonella typhimurium and Shigella dysenteriae have their own distinct metabolic profiles not aligning with lysine positive, arginine negative, and ornithine positive. Staphylococcus aureus and Streptococcus pyogenes are primarily Gram-positive cocci, which differ fundamentally in their