Which organism is a common member of the normal biota of the upper respiratory tract?

• A. Corynebacterium jeikeium
• B. Lactobacillus
• C. Staphylococcus epidermidis
• D. Viridans streptococcus

Answer: (D)

Normal flora in the upper airway includes viridans group streptococci, which commonly colonize the mouth and oropharynx as harmless, resident microbes. They are adapted to the oral environment and form part of the stable commensal community that helps keep other, potentially pathogenic organisms in check. This makes them a classic example of the typical upper respiratory tract biota.

The other organisms listed are more characteristic of other habitats: Corynebacterium jeikeium is primarily associated with skin/mucous membranes and is notable in hospital settings; Lactobacillus mainly resides in the gastrointestinal tract and female genital tract; Staphylococcus epidermidis is a skin commensal and not a major, consistent resident of the upper airway.