Mechanism of Lactase Deficiency

Pediatric essentials · 10 single-best-answer MCQs · Select one option in each card

Sheet 1 — Core mechanism5 MCQs · 1 recall panel

MCQ 01

Where is lactase located?

Lactase is a brush-border disaccharidase on small-intestinal enterocytes, especially villous cells.

MCQ 02

Lactase normally hydrolyses lactose into:

This is the key enzymatic step: lactose → glucose + galactose for absorption.

MCQ 03

Why does unabsorbed lactose cause watery diarrhoea?

Undigested lactose remains in the lumen as an osmotically active solute and pulls water into the bowel.

MCQ 04

Colonic bacteria ferment lactose mainly producing:

Fermentation produces hydrogen, carbon dioxide and short-chain fatty acids—causing gas, bloating and acidic stools.

MCQ 05

The best mechanism for lactose intolerance symptoms is:

Core chain: low lactase → lactose malabsorption → osmotic water retention + bacterial fermentation.

MECHANISM MAP↓ lactase
→ lactose stays in lumen
→ osmotic diarrhoea
+ bacterial gas/acid

Exam anchor: lactose intolerance is a carbohydrate digestion problem—not cow-milk protein allergy.

Sheet 2 — Types & causes5 MCQs · 1 recall panel

MCQ 06

Congenital lactase deficiency is usually:

Congenital lactase deficiency is a rare inherited disorder with very low or absent activity from birth.

MCQ 07

Primary lactase non-persistence results from:

After weaning, genetically programmed downregulation of LCT expression reduces lactase activity.

MCQ 08

A common cause of secondary lactase deficiency is:

Mucosal injury damages villous brush-border lactase; activity often returns after the mucosa heals.

MCQ 09

Why are preterm infants prone to lactose malabsorption?

Lactase activity rises late in fetal life, so prematurity can cause transient developmental deficiency.

MCQ 10

A hydrogen breath test becomes positive because:

Unabsorbed lactose reaches bacteria; hydrogen enters blood and is measured in exhaled breath.

TYPE TRIOCongenital = from birth
Primary = programmed decline
Secondary = mucosal injury

Exam trap: secondary lactase deficiency is often temporary; treat the underlying mucosal disease.

Lactase Mechanism Sheets