What is the most likely diagnosis for a 13-year-old presenting with nausea, vomiting, and loose stools, with vomiting occurring one hour after eating?

Most Likely Diagnosis: Acute Viral Gastroenteritis

The most likely diagnosis for this 13-year-old is acute viral gastroenteritis, which is the leading cause of acute vomiting, nausea, and loose stools in children and adolescents. 1 The clinical presentation of vomiting occurring one hour after eating, combined with nausea and loose stools, fits the classic pattern of acute gastroenteritis in this age group.

Why Viral Gastroenteritis is Most Likely

• Watery diarrhea and vomiting in a child most likely represent viral gastroenteritis and therefore do not require antimicrobial therapy. 1 This is the most common cause of acute vomiting in children. 2

• The symptom triad of nausea, vomiting, and loose stools occurring together is pathognomonic for acute gastroenteritis, which accounts for 1.5 million office visits annually in U.S. children. 3

• Norovirus is the leading pathogen causing 58% of gastroenteritis cases in the United States, followed by Salmonella at 11%. 1

Important Alternative Diagnoses to Consider

Bacterial Food Poisoning (Less Likely but Important)

• If vomiting began exactly 1-4 hours after a specific meal, bacterial toxin-mediated food poisoning from Staphylococcus aureus or Bacillus cereus becomes a strong consideration. 4 The 1-hour incubation period is characteristic of preformed bacterial toxins. 4

• However, this typically presents with abrupt onset of vomiting and abdominal cramps within 1-4 hours of eating contaminated food, with diarrhea developing later at 5-10 hours. 4, 5 The question states vomiting occurs "one hour after they eat" which could mean after each meal (suggesting gastroenteritis) or one hour after a specific contaminated meal (suggesting food poisoning).

Food Protein-Induced Enterocolitis Syndrome (FPIES)

• FPIES should be considered if there is repetitive vomiting 1-4 hours after ingestion of a specific food, especially with recent introduction of new foods. 1, 6

• Additional features supporting FPIES include extreme lethargy, marked pallor, hypotension, and diarrhea typically occurring 5-10 hours after ingestion. 1, 6

• This diagnosis is less likely in a 13-year-old unless there is a clear temporal relationship to a specific new food trigger, as FPIES more commonly presents in infancy. 1

Critical Red Flags Requiring Immediate Evaluation

You must rule out life-threatening conditions before settling on viral gastroenteritis:

• Bilious or bloody vomiting suggests intestinal obstruction, malrotation with volvulus, or intussusception. 2

• Altered sensorium, extreme lethargy, or toxic appearance may indicate sepsis, meningitis, metabolic disorder, or severe dehydration requiring IV resuscitation. 6, 2

• Severe abdominal pain with bent-over posture raises concern for appendicitis in this age group. 2

• Signs of severe dehydration (≥10% fluid deficit): prolonged skin tenting >2 seconds, cool extremities, decreased capillary refill, rapid deep breathing. 1

Diagnostic Approach

• Physical examination is the best way to evaluate hydration status using the Clinical Dehydration Scale based on appearance, eyes, mucous membranes, and tears. 3

• Stool microbiological tests are not routinely needed when viral gastroenteritis is the likely diagnosis in children with mild illness. 3

• Laboratory testing (CBC, electrolytes, blood gases) is indicated only if there are red flag signs, moderate-to-severe dehydration, or concern for non-GI causes like diabetic ketoacidosis or metabolic disorders. 2, 3

Management Priorities

• Oral rehydration therapy is the mainstay of treatment for mild-to-moderate dehydration. 5, 3 Half-strength apple juice followed by preferred liquids is as effective as IV rehydration for preventing hospitalization. 3

• Ondansetron 0.15 mg/kg IV or 0.2 mg/kg oral (maximum 16 mg) may be given if persistent vomiting prevents oral rehydration. 6, 2, 3

• Resume normal age-appropriate diet immediately after rehydration is completed. 5

• Antimotility drugs like loperamide should NOT be used in children under 18 years. 5

Common Pitfalls to Avoid

• Do not assume viral gastroenteritis without considering other serious conditions, especially meningitis, sepsis, appendicitis, or metabolic disorders that can present with vomiting. 1, 6, 2

• Do not delay fluid resuscitation while waiting for laboratory results if the child appears significantly lethargic or dehydrated. 6

• Do not use routine antidiarrheal agents as they do not reduce diarrhea volume or duration and can cause serious side effects including ileus and drowsiness. 1