A 12-month-old boy presents for a routine checkup. The patient immigrated from the Philippines with his parents a few months ago. No prior immunization records are available. The patient’s mother claims that he had a series of shots at 6 months of age which gave him a severe allergic reaction with swelling of the tongue and the face. She also remembers that he had the same reaction when she introduced solid foods to his diet, including carrots, eggs, and bananas. Which of the following vaccinations are not recommended for this patient?

Intramuscular influenza vaccine

Measles, mumps, and rubella (MMR) vaccine

A 6-year-old boy is brought to the emergency room by ambulance, accompanied by his kindergarten teacher. Emergency department staff attempt to call his parents, but they cannot be reached. The boy’s medical history is unknown. According to his teacher, the boy was eating in the cafeteria with friends when he suddenly complained of itching and developed a widespread rash. Physical exam is notable for diffuse hives and tongue edema. His pulse is 100/min and blood pressure is 90/60 mmHg. The boy appears frightened and tells you that he does not want any treatment until his parents arrive. Which of the following is the next best step in the management of this patient?

Immediately administer epinephrine and provide supportive care

Continue calling the patient’s parents and do not intubate until verbal consent is obtained over the phone

Obtain written consent to intubate from the patient’s teacher

Wait for the patient's parents to arrive, calm the patient, and provide written consent before intubating

Obtain written consent to intubate from the patient

An 82-year-old woman presents with 2 months of foul-smelling, greasy diarrhea. She says that she also has felt very tired recently and has had some associated bloating and flatus. She denies any recent abdominal pain, nausea, melena, hematochezia, or vomiting. She also denies any history of recent travel and states that her home has city water. Which of the following tests would be most appropriate to initially work up the most likely diagnosis in this patient?

Tissue transglutaminase antibody test

CT of the abdomen with oral contrast

An 11-month-old boy is brought to a pediatrician by his parents for evaluation of vomiting and watery diarrhea over the last day. The mother informs the pediatrician that the boy had consumed an apple bought from a fruit vendor on the previous day, but that otherwise there has been no recent change in his diet. There is no history of blood in the stool, flatulence, irritability, or poor appetite. There is no history of recurrent or chronic diarrhea or any other gastrointestinal symptoms. On physical examination, his temperature is 37.6°C (99.6°F), pulse is 120/min, respirations are 24/min, and blood pressure is 92/60 mm Hg. General examination reveals a playful infant with normal skin turgor and no sunken eyes. The pediatrician explains to the parents that he most likely has acute gastroenteritis and that no specific medication is indicated at present. He also instructs the parents about his diet during the illness and reviews the danger signs of dehydration. He suggests a follow-up evaluation after 48 hours or earlier if any complications arise. Which of the following dietary recommendations did the pediatrician make?

Plenty of juices and carbonated sodas

A 7-year-old boy presents to an urgent care clinic from his friend’s birthday party after experiencing trouble breathing. His father explains that the patient had eaten peanut butter at the party, and soon after, he developed facial flushing and began scratching his face and neck. This has never happened before but his father says that they have avoided peanuts and peanut butter in the past because they were worried about their son having an allergic reaction. The patient has no significant medical history and takes no medications. His blood pressure is 94/62 mm Hg, heart rate is 125/min, and respiratory rate is 22/min. On physical examination, his lips are edematous and he has severe audible stridor. Of the following, which type of hypersensitivity reaction is this patient experiencing?

Type I hypersensitivity reaction

Type II hypersensitivity reaction

Type III hypersensitivity reaction

Type IV hypersensitivity reaction

Combined type I and type III hypersensitivity reactions

A 5-year-old is brought into the emergency department for trouble breathing. He was at a family picnic playing when his symptoms began. The patient is currently struggling to breathe and has red, warm extremities. The patient has an unknown medical history and his only medications include herbs that his parents give him. His temperature is 99.5°F (37.5°C), pulse is 112/min, blood pressure is 70/40 mmHg, respirations are 18/min, and oxygen saturation is 82% on 100% O2. Which of the following is the best initial step in management?

Albuterol, ipratropium, and magnesium

You are working in the emergency room of a children's hospital when a 4-year-old girl is brought in by ambulance due to "difficulty breathing." The patient had been eating lunch on a school field trip when she suddenly complained of abdominal pain. Shortly thereafter, she was noted to have swelling of the lips, a rapidly developing red rash and difficulty breathing. In the ambulance her blood pressure was persistently 80/50 mmHg despite intramuscular epinephrine. In the course of stabilization and work up of the patient, you note an elevated tryptase level. What is the mechanism behind this elevated tryptase level?

Cross-linking of IgE on mast cells

IgM mediated complement activation

Antibody-antigen immune complexes

Cross-linking of IgG on mast cells

A 4-week-old infant is brought to the physician by his mother because of blood-tinged stools for 3 days. He has also been passing whitish mucoid strings with the stools during this period. He was delivered at 38 weeks' gestation by lower segment transverse cesarean section because of a nonreassuring fetal heart rate. He was monitored in the intensive care unit for a day prior to being discharged. His 6-year-old brother was treated for viral gastroenteritis one week ago. The patient is exclusively breastfed. He is at the 50th percentile for height and 60th percentile for weight. He appears healthy and active. His vital signs are within normal limits. Examination shows a soft and nontender abdomen. The liver is palpated just below the right costal margin. The remainder of the examination shows no abnormalities. Test of the stool for occult blood is positive. A complete blood count and serum concentrations of electrolytes and creatinine are within the reference range. Which of the following is the most appropriate next step in management?

Continue breastfeeding and advise mother to avoid dairy and soy products

Perform stool antigen immunoassay

Perform an air enema on the infant

Assess for IgA (anti‑)tissue transglutaminase antibodies (tTG)

Stop breastfeeding and switch to soy-based formula

Food allergies and intolerances for USMLE Step 3: High-Yield Pediatrics Lessons

Allergy vs. Intolerance - The Great Mix-Up

Feature	Food Allergy	Food Intolerance
Mechanism	Immune-mediated (IgE/Non-IgE)	Non-immune (e.g., enzyme defect)
Onset	Rapid (mins to 2 hrs)	Gradual (hours to days)
Amount	Minute quantity	Dose-dependent
Symptoms	Systemic, Anaphylaxis possible	Primarily GI (bloating, diarrhea)

Food allergy sensitization immune mechanism

Common Culprits & Features - The Usual Suspects

Infancy & Early Childhood: Cow's milk, soy, eggs, wheat.
Older Children & Adults: Peanuts, tree nuts (cashews, walnuts), fish, shellfish.

Reaction Types:

IgE-Mediated (Rapid Onset): Urticaria, angioedema, wheezing, vomiting, anaphylaxis within minutes to <2 hours.
Non-IgE-Mediated (Delayed): Chronic diarrhea, vomiting, FPIES (Food Protein-Induced Enterocolitis Syndrome), eczema flare. Onset >2 hours to days.

The Big 8 Food Allergens Infographic

⭐ Cow's milk protein allergy (CMPA) is the most common food allergy in infants and young children, presenting with both IgE and non-IgE mediated symptoms.

Diagnostic Workup - The Allergy Detective

Allergy-focused history: The cornerstone. Inquire about timing, symptoms, reproducibility, and dietary details.
Skin Prick Test (SPT):
- Wheal >3 mm than negative control is positive.
- Quick, sensitive, but risk of false positives.
Specific IgE (sIgE) Test:
- Blood test; positive if >0.35 kU/L.
- Safer if risk of anaphylaxis with SPT.

Skin Prick Test for Allergies

⭐ Gold Standard: The Double-Blind, Placebo-Controlled Food Challenge (DBPCFC) is the definitive method for diagnosing a food allergy.

Key Syndromes - The Special Cases

FPIES (Food Protein-Induced Enterocolitis Syndrome):
- Non-IgE mediated; presents at 2-6 months.
- Profuse, repetitive vomiting 1-4 hrs post-ingestion → dehydration, lethargy, shock.
- Triggers: Cow's milk, soy, rice, oats.
Heiner Syndrome:
- Food hypersensitivity pulmonary hemosiderosis (rare).
- Triad: Pulmonary infiltrates, hemosiderosis (hemoptysis), and iron deficiency anemia with cow's milk allergy.
Oral Allergy Syndrome (OAS):
- Pollen-food allergy; IgE-mediated.
- Itching/tingling of mouth/throat with raw fruits/veg.

⭐ FPIES is a clinical diagnosis; allergy tests (skin prick, specific IgE) are typically negative.

Oral Allergy Syndrome: Pollen-Food Cross-Reactivity Chart

Management & Prevention - The Action Plan

Core Strategy: Strict dietary avoidance of the offending food allergen.
Written Emergency Plan: Essential for patients, families, and schools.
Anaphylaxis: Immediate intramuscular Epinephrine (0.01 mg/kg of 1:1000, max 0.5 mg) is life-saving.
Oral Immunotherapy (OIT): Medically supervised desensitization is an emerging option.

⭐ A biphasic reaction (second wave of anaphylaxis) can occur 1-72 hours after the initial reaction, requiring prolonged observation.

Epinephrine auto-injector use demonstration

High‑Yield Points - ⚡ Biggest Takeaways

Cow's milk protein allergy (CMPA) is the most common food allergy in infancy, typically resolving by age 5.

Most reactions are IgE-mediated (Type I hypersensitivity), with rapid onset of urticaria, angioedema, or anaphylaxis.

The gold standard for diagnosis is a physician-supervised Oral Food Challenge (OFC).

Food Protein-Induced Enterocolitis Syndrome (FPIES) is a severe, non-IgE-mediated gut allergy causing delayed vomiting and lethargy.

Lactose intolerance is a non-immune reaction due to lactase deficiency, diagnosed via a hydrogen breath test.

Strict allergen avoidance is key; epinephrine is the first-line treatment for anaphylaxis.

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