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

An erythematous wheal is a localized skin reaction that can occur after an allergy injection, typically indicating a mild allergic response. While it may cause discomfort, it is not life-threatening and is generally self-limiting, distinguishing it from more severe systemic reactions.

Pruritus, or itching at the injection site, is another common and mild reaction that often occurs after allergy injections. This symptom usually resolves quickly without medical intervention and does not indicate a serious adverse effect, making it less concerning than systemic reactions.

Sneezing is generally a mild symptom associated with allergies and may occur in response to allergens. It does not typically signify a severe reaction to an allergy injection and is often not a cause for alarm, as it is manageable and transient.

Ulceration at the injection site can occur due to local irritation or infection but is not directly related to the systemic effects of the allergy injection itself. While it may require attention, it is not as serious or urgent as symptoms like wheezing and light-headedness.