Final answer:
The appropriate monotherapy for suspected cellulitis from a lake exposure is likely an antibiotic like penicillin, after performing tests to detect the specific pathogen, such as streptococcal bacteria.
Step-by-step explanation:
For a patient with suspected cellulitis on their back, particularly after coming back from an environment like a lake, monotherapy would typically involve the administration of an antibiotic to combat the likely bacterial cause. Since cellulitis is often due to streptococcal infections, penicillin is commonly prescribed since streptococci have not developed widespread resistance to it. The treatment is chosen based on the pathogen's susceptibility to antibiotics. It is important to use immunological tests, such as agglutination reactions or ELISA (Enzyme-Linked Immunosorbent Assay), to detect the specific type of streptococcal bacteria present. If a patient presented with erythema nodosum, however, it is self-limiting and the recommended treatment may not include antibiotics but rather NSAIDs, cool wet compresses, elevation, and bed rest.