Differential Diagnosis

The patient's symptoms of repeated episodes of watery diarrhea, flatulence, abdominal discomfort following meals, and significant weight loss over a year and a half, along with the characteristics of the stool sample, guide the differential diagnosis.

• Single Most Likely Diagnosis

  • Malabsorption due to Lactose Intolerance or other Carbohydrate Malabsorption: The patient's symptoms of watery diarrhea, flatulence, and abdominal discomfort after meals, along with the finding of frothy, foul-smelling stools that float, are highly suggestive of malabsorption of carbohydrates, most commonly lactose. The fecal osmolar gap of 95 mOsm/kg supports this diagnosis, as it indicates the presence of osmotically active substances (like unabsorbed carbohydrates) in the stool.

• Other Likely Diagnoses

  • Irritable Bowel Syndrome (IBS): Although IBS can cause chronic diarrhea and abdominal discomfort, the significant weight loss and the specific characteristics of the stool (frothy, foul-smelling, and floating) are less typical for IBS.
  • Small Intestine Bacterial Overgrowth (SIBO): SIBO can lead to malabsorption, diarrhea, and weight loss. The symptoms and the nature of the stools could fit SIBO, especially if there's an underlying condition affecting the small intestine's motility or anatomy.
  • Celiac Disease: An autoimmune reaction to gluten leading to small intestine mucosal damage, causing malabsorption. However, the absence of other typical symptoms like fatigue, and the specific stool characteristics, makes it less likely but still a consideration.

• Do Not Miss Diagnoses

  • Giardiasis: Despite the microscopy showing no evidence of cysts or trophozoites, giardiasis can cause chronic diarrhea, weight loss, and malabsorption. It's crucial to consider this, especially if the patient has been exposed to contaminated water or has traveled, even though the history does not suggest this.
  • Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic diarrhea, weight loss, and abdominal pain. The absence of blood in the stool does not rule out IBD, as Crohn's disease, in particular, can present with non-bloody diarrhea.
  • Colorectal Cancer: Although rare in a 30-year-old, unexplained weight loss and chronic diarrhea could be symptoms of colorectal cancer, especially if there's a family history, which is not mentioned but could be overlooked.

• Rare Diagnoses

  • Short Bowel Syndrome: A condition resulting from a significant resection of the small intestine, leading to malabsorption. The history does not suggest previous surgeries, but it's a consideration in severe malabsorption cases.
  • Congenital Sucrase-Isomaltase Deficiency: A rare genetic disorder leading to the inability to digest certain carbohydrates, resulting in chronic diarrhea and malabsorption.
  • Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, diarrhea, weight loss, and abdominal pain, among other symptoms. It's very uncommon and would typically present with additional systemic symptoms.