Which of the following agents is most helpful for controlling a gram-positive bacterial infection in a patient allergic to penicillin?

Cephalexin (Keflex) is the most helpful agent for controlling a gram-positive bacterial infection in a patient allergic to penicillin because it belongs to a class of antibiotics known as cephalosporins. Cephalosporins have a similar structure to penicillin, but they are often less likely to provoke an allergic reaction in individuals who are allergic to penicillin.

Patients with a history of penicillin allergy may benefit from cephalosporins, particularly first-generation cephalosporins like cephalexin, which are effective against gram-positive bacteria. This makes cephalexin a good choice for treating infections caused by organisms where penicillin is typically effective, but without the risk of an allergic reaction.

The other agents listed may not be suitable or effective for this scenario. Gentamicin is an aminoglycoside and is primarily effective against gram-negative bacteria rather than gram-positive. Methicillin, while effective against certain gram-positive bacteria, is a penicillin derivative and can trigger allergic reactions in individuals with penicillin allergy. Carbenicillin, also a penicillin derivative, is similarly associated with allergic reactions and is not the best choice due to its structure. Thus, cephalexin serves as a safer and effective