Question

a doctor is able to isolate and grow a bacterium that is causing an infection in a patient. the doctor prescribes penicillin, and when the patient returns one week later, the patient demonstrates no signs of improvement. this infection is likely caused by:

• a virus.
• gram-positive bacteria.
• gram-negative bacteria.
• a fungus.

Explanation:

Penicillin targets bacterial cell walls, so it is ineffective against viruses (which lack cell walls). The doctor isolated a bacterium, but if the infection persisted despite penicillin, the actual cause might be a virus (since penicillin doesn’t work on viruses). Gram-positive bacteria are typically susceptible to penicillin, while Gram-negative have an outer membrane that can limit penicillin’s effectiveness, but the key point here is that penicillin has no effect on viruses. Fungi are treated with antifungals, not penicillin. Thus, the most likely cause is a virus.

Answer:

a virus.