A 4-week-old boy presents to the pediatrician for vomiting. The patient is breastfed and typically feeds for 20-30 minutes every 2 hours. One week ago, he began vomiting and regurgitating breastmilk through his nose after most feedings. His stools over the past week have also become blood-streaked. The patient does not seem to be in any distress when he passes these bloody stools. The patient surpassed his birth weight by 2 weeks of age but has fallen 1 standard deviation on the growth curve since then. His temperature is 98.6°F (37°C), blood pressure is 78/47 mmHg, pulse is 115/min, and respirations are 28/min. On physical exam, the patient is well-appearing. His face and back have the physical exam finding seen in Figure A. The patient’s abdomen is soft, non-tender, and non-distended. A digital rectal exam reveals a small amount of blood in the rectal vault. Which of the following is the most appropriate next step in management?
Obtain abdominal ultrasound
Initiate a proton pump inhibitor
Switch to hydrolyzed formula
Modify mother's diet
Modify mother's diet
This infant presents with vomiting, bloody stools, and poor weight gain, which suggests a diagnosis of milk-protein allergy. The most appropriate next step in management is the modification of the mother’s diet to eliminate cow’s milk and soy.

Infant food protein-induced proctocolitis is caused by distal colon inflammation secondary to an immune reaction to certain food proteins. Milk-protein allergy is the most common food allergy in children and presents with vomiting, bloody stools, and poor weight gain or failure to thrive. In an infant that is breastfed, the most appropriate next step in management is the elimination of cow’s milk and soy from the mother’s diet. In an infant who is formula fed, the most appropriate next step would be switching to either an extensively hydrolyzed or amino acid-based infant formula.

Giannetti et al. review the evidence regarding the diagnosis and treatment of milk protein allergy in infants. They discuss how the selective elimination of certain proteins from the diet is usually effective. They recommend considering oral immunotherapy as an adjuvant in the treatment of this disease.

Figure/Illustration A is a clinical photograph that demonstrates the finding of eczema (red circle). This finding is commonly found in patients with milk-protein allergy.

Incorrect Answers:
Answer A: Initiating a proton pump inhibitor would be appropriate in patients with refractory gastroesophageal reflux disease, which presents with regurgitation or vomiting and symptoms of feeding aversion or failure to thrive. Because this patient presents with bloody stools, he is more likely to have a milk protein allergy.

Answer B: Obtaining an abdominal ultrasound would help evaluate for pyloric stenosis, which presents in infants between 3-6 months of age with non-bilious, projectile vomiting. Pyloric stenosis does not cause bloody stools. Treatment is with surgical pyloromyotomy.

Answer C: Providing reassurance would not be appropriate for an infant presenting with bloody stools and evidence of poor weight gain. The cause of his blood-streaked stools should be investigated to ensure this patient is getting the nutrition he needs.

Answer E: Switching to a hydrolyzed formula would be appropriate if this patient were formula fed, but it is not necessary to discontinue breastfeeding if the mother is willing to modify her diet to exclude cow’s milk and soy.

Bullet Summary:
A milk-protein allergy in a breastfed infant should be treated with modification of the mother’s diet to eliminate cow’s milk and soy.