WBR0467

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D. and Alison Leibowitz [1])]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::Gastrointestinal
Prompt	[[Prompt::A 6-year-old European boy is brought to the physician's office with complaints of abdominal pain, bloating, and explosive diarrhea following milk ingestion. The patient's mother explains that he used to drink milk during infancy, and his symptoms are relatively new. Breath hydrogen levels are elevated, and jejunal biopsies demonstrate normal gastrointestinal (GI) mucosa. Which of the following most likely characterizes the type of diarrhea in this patient?]]
Answer A	[[AnswerA::Increase in fluid secretion in the small bowel lumen due to decreased absorption of electrolytes with stool osmolal gap < 50 mOsm/kg.]]
Answer A Explanation	[[AnswerAExp::Secretory diarrhea is frequently characterized by increased fluid secretion in the small bowel lumen due to decreased absorption of electrolytes.]]
Answer B	AnswerB::Fecal fat output exceeding the absorptive capacity of the intestine
Answer B Explanation	AnswerBExp::Fatty diarrhea is characterized by fecal fat output exceeding the absorptive capacity of the intestine.
Answer C	AnswerC::Active solutes that obligate water retention in the intestinal lumen
Answer C Explanation	[[AnswerCExp::Osmotic diarrhea is characterized by the retention of water in the intestinal lumen due to the presence of active solutes. In osmotic diarrhea, the stool osmolal gap is usually > 100 mOsm/kg.]]
Answer D	AnswerD::Enterocyte damage that causes mucosal disruption and abnormal absorption
Answer D Explanation	AnswerDExp::Inflammatory diarrhea is characterized by enterocyte damage, which causes mucosal disruption and abnormal absorption.
Answer E	AnswerE::Altered small intestinal motor activity
Answer E Explanation	AnswerEExp::Motility disorders may cause diarrhea. Motility disorders are characterized by a limited time for appropriate digestion to occur. They are common in patients with hyperthyroidism.
Right Answer	RightAnswer::C
Explanation	[[Explanation::Lactase deficiency (hypolactasia) is the most common type of disaccharidase deficiency that is associated with milk intolerance. Lactase deficiency is common among North American, European, and Australian populations. The enzyme lactase-phloizin hydrolase (lactase) is a beta-galactosidase that hydrolyzes lactose to glucose plus galactose before they are absorbed by the intestinal enterocytes into the blood. The enzyme is normally present on the apical surface of enterocytes along the brush border of the small intestine, especially in the mid-jejunum. The activity of lactase is at its peak at birth and decreases gradually with age. Following milk ingestion, patients with lactase deficiency complain of abdominal pain, bloating, explosive foul-smelling diarrhea, and occasionally, headaches. Lactase deficiency exists in 3 forms: congenital (autosomal recessive disorder characterized by failure to thrive and infantile diarrhea), primary or lactose nonpersistence (gradual decline of lactase activity with age), and secondary (Potentially reversible loss of lactase activity in patients with lactase persistence following a GI illness). Diagnosis of lactase deficiency may be made by measuring blood glucose levels 30 minutes following oral ingestion of a lactose load (rise in glucose is typically < 20 mg/dL in lactase deficiency) or by a hydrogen breath test that measures breath hydrogen levels 3-6 hrs following oral ingestion of lactose ( A rise of > 20 ppm above baseline in lactase deficiency). Real-time PCR may be used to identify the lactase gene Jejunal biopsies have a low sensitivity and often demonstrate normal GI mucosa. Lactase deficiency manifests with osmotic-type diarrhea due to patients' inability to absorb lactose, a compound containing osmotically active metabolites, in the intestinal lumen. Metabolites drag water into the lumen, through osmosis, resulting in diarrhea. Accordingly, measurements of stool osmotic gap is a clinical test that helps physicians distinguish osmotic diarrhea from secretory diarrhea. In osmotic diarrhea, the stool osmolal gap is usually greater than 100 mOsm/kg; whereas secretory diarrhea is associated with a stool osmolal gap less than 50 mOsm/kg. Educational Objective: Lactase deficiency is a common cause of osmotic diarrhea that manifests with abdominal pain, and explosive, foul-smelling diarrhea following milk ingestion. Osmotic diarrhea is characterized by the retention of water in the intestinal lumen due to the presence of active solutes. In osmotic diarrhea, the stool osmolal gap is usually > 100 mOsm/kg. References: Corinaldesi R, Stanghellini V, Barbara G, et al. Clinical approach to diarrhea. Intern Emerg Med. 2012;7(Suppl 3):S255-S262. Lomer MCE, Parkes GC, Sanderson JD. Review article: lactose intolerance in clinical practice - myths and realities. Aliment Pharmacol Ther. 2007;27:93-103. First Aid 2014 page 353]]
Approved	Approved::Yes
Keyword	WBRKeyword::Osmotic diarrhea, WBRKeyword::Lactase deficiency, WBRKeyword::Lactase, WBRKeyword::Enzyme deficiency, WBRKeyword::Diarrhea, WBRKeyword::Disaccharide, WBRKeyword::Lactose intolerance, WBRKeyword::Hydrogen breath test, WBRKeyword::Types of diarrhea
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