Which of the following is the most serious adverse effect in a patient who receives an allergy injection?

An erythematous wheal is a localized reaction that can occur at the injection site, typically indicating a mild allergic response. While it may be uncomfortable, it is generally not serious and usually resolves without intervention.

Pruritus, or itching, at the injection site is also a common and mild reaction. Like the wheal, it signifies an immune response but does not indicate a severe or systemic allergic reaction and is not life-threatening.

Sneezing can occur as an allergic response but is generally considered a minor symptom. It does not suggest a serious adverse effect and usually resolves on its own without requiring medical attention.

Ulceration is an uncommon reaction and can indicate a localized issue with skin integrity; however, it does not reflect a systemic response. While it may require treatment, it is not as serious as wheezing and light-headedness, which reflect potential anaphylaxis.