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Lactase deficiency
Lactase breaks down lactose into glucose + galactose. It is a brush border disaccharidase.
| Your disaccharides:
Lactose = glucose + galactose Maltose = 2 x glucose Sucrose = glucose + fructose |
If the question asks you where the enzyme is located, it is on the surface of the enterocytes, predominantly in the jejunum.
| The brush border location of lactase is exceedingly HY. |
Intestinal biopsy shows NORMAL-appearing villi, or “benign mucosa.” There is nothing wrong with the villi appearance-wise in lactase deficiency.
In contrast, Celiac disease, which is gluten intolerance, has blunting of the villi on biopsy.
| Normal biopsy → lactase deficiency
Flattened villi → Celiac disease Not rocket science. But for some reason if people get asked about the biopsy finding in lactase deficiency they’re often deer in the headlights. |
A super-HY USMLE pearl is that gastroenteritis can lead to transient (secondary) lactose intolerance due to shedding/inflammation of intestinal epithelium, which is where the diasaccharidase resides.
This is most notably due to rotavirus infection in pediatrics.
The classic USMLE question will mention a kid with recent watery diarrhea (rotavirus) who, after having recovered, presents with new-onset osmotic diarrhea, bloating, and cramps following consumption of dairy products. This is self-limited lactase deficiency.
| Annoying, but the USMLE sometimes refers to lactose (not lactase) as galactosyl β-1,4-glucose. |
Sometimes the question will mention in the stem that a patient has been having diarrhea (they won’t mention dairy products), and that stool demonstrates ↓ pH and breath shows ↑ hydrogen content. This is important because hydrogen breath test and stool pH are diagnostic tests.
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