Differential Diagnosis

The patient's symptoms of repeated episodes of watery diarrhea, flatulence, abdominal discomfort following meals, and significant weight loss, along with the presence of frothy, foul-smelling stools that float, suggest malabsorption. The absence of blood in the stool and the lack of evidence for parasitic infections help narrow the differential.

• Single Most Likely Diagnosis
  • Celiac Disease: This condition is characterized by malabsorption due to an immune reaction to gluten, leading to symptoms like diarrhea, weight loss, and abdominal discomfort. The presence of angular stomatitis and a low BMI further supports this diagnosis, as they can be signs of malnutrition. The fecal osmolar gap is also consistent with a malabsorptive process.
• Other Likely Diagnoses
  • Lactose Intolerance: This is a common cause of malabsorption leading to similar gastrointestinal symptoms. The patient's symptoms following meals and the characteristics of the stool could be consistent with lactose intolerance.
  • Small Intestine Bacterial Overgrowth (SIBO): SIBO can cause malabsorption, leading to diarrhea, bloating, and weight loss. The presence of frothy, foul-smelling stools could indicate an overgrowth of bacteria in the small intestine.
• Do Not Miss Diagnoses
  • Crohn's Disease: Although less likely given the absence of blood in the stool and no significant abdominal tenderness, Crohn's disease can cause malabsorption and significant weight loss. It's crucial to consider this diagnosis due to its potential severity and the need for early intervention.
  • Giardiasis: Despite the microscopy showing no evidence of cysts or trophozoites, giardiasis can sometimes be missed on initial stool exams. It's a significant cause of malabsorptive diarrhea and should be considered, especially with the patient's symptoms of weight loss and malabsorption.
• Rare Diagnoses
  • Tropical Sprue: This is a rare condition causing malabsorption, typically seen in individuals who have traveled to or lived in tropical areas. Given the patient's lack of travel history, this is less likely but remains a consideration in the differential diagnosis of chronic diarrhea and malabsorption.
  • Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, weight loss, and diarrhea. It's essential to consider due to its severity and the need for specific antibiotic treatment, although it is much less common than other causes of malabsorption.