Differential Diagnosis for Blood in Mucus of a 5-Month-Old Baby

Single Most Likely Diagnosis

• Allergic Proctocolitis: This condition is characterized by the presence of blood in the stool, often due to a milk protein allergy, even in breastfed babies, as small amounts of the protein can pass through the mother's milk. The fact that the issue persists despite a dairy, soya, and egg-free diet suggests the allergy might be to another protein or that the mother's diet needs further adjustment.

Other Likely Diagnoses

• Gastroesophageal Reflux Disease (GERD): Although more commonly associated with spit-up and discomfort, GERD can sometimes cause small amounts of blood in the stool due to irritation of the esophagus or stomach.
• Anal Fissure: A small tear in the anal lining can cause blood to appear in the stool, often accompanied by pain during bowel movements, though in infants, pain might not be as evident.
• Infection: Bacterial or viral infections can cause gastrointestinal symptoms, including blood in the stool, though other symptoms like fever, vomiting, or diarrhea are usually present.

Do Not Miss Diagnoses

• Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood flow. It's a medical emergency and can cause bloody stools, though typically accompanied by severe abdominal pain, vomiting, and a palpable abdominal mass.
• Hirschsprung's Disease: A congenital condition affecting the large intestine, leading to severe constipation or intestinal obstruction. Bloody stools can be a symptom, along with failure to pass meconium in the first 48 hours of life, chronic constipation, and abdominal distension.
• Inflammatory Bowel Disease (IBD): Though rare in infants, conditions like Crohn's disease or ulcerative colitis can cause chronic diarrhea, blood in the stool, abdominal pain, and weight loss.

Rare Diagnoses

• Congenital vascular malformations: Abnormalities in blood vessels within the gastrointestinal tract can cause bleeding.
• Gastrointestinal duplication cysts: Rare congenital anomalies that can cause bleeding, obstruction, or infection.
• Meckel's Diverticulum: A small bulge in the small intestine that can cause bleeding due to the presence of stomach tissue that can secrete acid.

Each of these diagnoses requires careful consideration of the clinical presentation, and further diagnostic testing may be necessary to determine the underlying cause of the blood in the mucus of the 5-month-old baby.