A patient prescribed amoxicillin for an odontogenic infection develops a reaction after several days of therapy. Which presentation is MOST commonly seen?

Correct answer: B. A generalised maculopapular (morbilliform) rash

• A. SERUM SICKNESS is a type III immune-complex reaction with fever, arthralgia and urticaria at 7–14 days. It is genuinely rare with amoxicillin.
• B. CORRECT. The commonest immunological reaction to aminopenicillins is a delayed, non-urticarial MACULOPAPULAR or MORBILLIFORM RASH appearing after several days of therapy. It is a T-cell mediated (type IV) reaction, not IgE-mediated, and it is not itself life-threatening. It is strikingly common when amoxicillin is given during EBV infectious mononucleosis, where the rash rate approaches the majority of patients and does not indicate true penicillin allergy.
• C. STEVENS–JOHNSON SYNDROME is a severe mucocutaneous reaction with epidermal detachment and mucosal ulceration. It is a recognised but RARE drug reaction — the question asks for the most common presentation.
• D. ANAPHYLAXIS is the most FEARED penicillin reaction and the one that dictates emergency preparedness, but it is uncommon in absolute terms and is immediate (minutes), not delayed by days.

Note: Urticaria is IgE-mediated and does signal true allergy, but the single most common amoxicillin skin reaction is the delayed non-urticarial MACULOPAPULAR rash — and distinguishing the two changes management, since a morbilliform rash does not necessarily preclude future beta-lactam use whereas urticaria and anaphylaxis do. If the exam offers 'nausea and vomiting', note that GI upset is the most common ADVERSE EFFECT overall, though not an allergic reaction.