USMLE Step 1 Question 302

Question:

Following a fad diet meal of skim milk, an adult female patient experiences abdominal distention, nausea, cramping, and pain followed by a watery diarrhea. What condition is most likely present?

Correct Answer: A.  Lactase deficiency

In many populations, a majority of adults are deficient in lactase (A) and hence intolerant to the lactose in milk. With no transporter existing for lactose in its disaccharide form, and no lactase enzyme to break it down into galactose and glucose, lactose accumulates in the small intestine. This creates an osmotic effect, leading to an outflow of water into the gut. This causes the clinical symptoms of bloating and diarrhea.

Steatorrhea (E), or fatty stools, is caused by unabsorbed fat, which can occur following a fatty meal in persons with a deficiency of pancreatic lipase. Lipoprotein lipase deficiency leads to hypertriglyceridemia. Sialidase deficiency (D) causes accumulation of sialic acid–containing proteoglycans and neurodegeneration.