A genital specimen inoculated into 10 B broth shows an alkaline reaction after overnight incubation without turbidity. What is the most likely explanation?

• A. pH change due to molecules in the clinical specimen
• B. Presence of Mycoplasma genitalium
• C. Presence of Mycoplasma hominis
• D. Presence of Ureaplasma urealyticum

Answer: (D)

The key idea is that some genital tract bacteria change the pH of broth by producing ammonia through substrate metabolism, and that can show up as an alkaline indicator even when visible growth isn’t seen. In a medium designed with urea, a urease-producing organism will rapidly hydrolyze urea to ammonia, raising the pH and turning the indicator alkaline. If you see an alkaline reaction after overnight without any turbidity, it points to organisms that can cause substantial pH change with little biomass. Ureaplasma urealyticum fits this pattern well because it is urease-positive and can alkalinize the medium quickly even when growth is sparse, due to ammonia production from urea. The other options don’t fit as cleanly: those organisms either rely on different substrates (for example, arginine metabolism) or don’t produce the same rapid urease-driven pH shift in a urea-containing medium, so the observed alkaline result with little visible growth most strongly supports the presence of Ureaplasma urealyticum.