Which of the following could be a sign of a medication-induced allergic reaction?

Swelling of the face is a classic sign of an allergic reaction, particularly in the context of medication-induced responses. This swelling, often referred to as angioedema, can occur as part of the body's immune response to a substance it mistakenly identifies as harmful. When medications are introduced into the system, they can trigger this immune response, leading to symptoms like facial swelling.

In contrast, decreased heart rate, lethargy, and weight gain are not typically associated with allergic reactions. A decreased heart rate may indicate a different type of physiological response, such as a side effect of certain medications on the cardiovascular system. Lethargy can occur due to various factors, including the sedative effects of some medications or mental health conditions, but it does not specifically indicate an allergic reaction. Weight gain can result from different causes, such as hormonal changes or lifestyle factors, and does not point directly to an allergy.

Therefore, among the options provided, swelling of the face is the most indicative of a medication-induced allergic reaction, distinguishing it from the other physiological responses that do not align with allergy symptoms.