This patient has small intestinal bacterial overgrowth (SIBO) presenting with bloating and diarrhea.  SIBO is caused by bacterial proliferation (eg, Escherichia coli, Bacteroides, Aeromonas) in the small bowel.  It is a common complication of scleroderma, which can alter intestinal motility due to smooth muscle atrophy and fibrosis.  As in this patient, symptoms often improve with antibiotics.

SIBO is characterized by mucosal enterocyte injury due to bacterial adherence, invasion, and toxin injury.  The activity of brush border enzymes is often impaired.  These changes lead to maldigestion and often malabsorption in more severe cases.  Specific macronutrient effects include the following:

• Carbohydrates:  decreased digestion and absorption of carbohydrates facilitate bacterial fermentation, producing carbon dioxide, hydrogen, and methane gases (eg, bloating).

• Fatty acids:  conjugated bile acids normally emulsify dietary fats in the proximal small bowel and are reabsorbed in the terminal ileum.  However, SIBO causes deconjugation of bile acids, leading to early bile acid resorption in the jejunum, reduced lipid emulsification, and fat malabsorption.

• Amino acids:  SIBO causes maldigestion of proteins and malabsorption of amino acids, occasionally leading to protein-losing enteropathy in severe cases.

(Choice A)  Defective apoprotein structure occurs in abetalipoproteinemia, an autosomal recessive disorder that presents in infancy with fat malabsorption and growth abnormalities; symptoms would not improve with antibiotics.

(Choice B)  Impaired release of pancreatic lipase occurs with chronic pancreatitis.  Patients typically have severe postprandial abdominal pain and steatorrhea (bulky, fat-laden stools); therapy involves pancreatic enzyme replacement, not antibiotics.

(Choice D)  Gastrin-secreting tumors (ie, Zollinger-Ellison syndrome) cause gastric acid hypersecretion, which lowers duodenal pH and inactivates pancreatic lipase.  Although it can cause diarrhea and fat malabsorption, hypersecretion typically presents with abdominal pain and multiple gastrointestinal ulcers; antibiotics are ineffective.

Educational objective:
Small intestinal bacterial overgrowth is characterized by bacterial proliferation in the small bowel, leading to mucosal inflammation with subsequent fermentation of carbohydrates and fat malabsorption; it manifests with diarrhea, abdominal distension, and flatulence.  Patients with impaired intestinal peristalsis (eg, systemic sclerosis) are at increased risk.  Treatment includes oral antibiotics.