USMLE Step 1 Gastrointestinal Practice Questions

Which cells in the stomach are responsible for secreting pepsinogen?

• (A) Chief cells
• (B) Parietal cells
• (C) G cells
• (D) Mucous neck cells

Bile salts, synthesized in the liver and stored in the gallbladder, are essential for the digestion and absorption of which nutrient class?

• (A) Fats
• (B) Proteins
• (C) Carbohydrates
• (D) Nucleic acids

Which hormone, released by S cells in the duodenum in response to low pH, stimulates pancreatic duct cells to secrete bicarbonate-rich fluid?

• (A) Secretin
• (B) Cholecystokinin (CCK)
• (C) Gastrin
• (D) Motilin

The absorption of vitamin B12 (cobalamin) in the terminal ileum requires binding to which protein, secreted by gastric parietal cells?

• (A) Intrinsic factor
• (B) Haptocorrin (R-protein)
• (C) Transcobalamin II
• (D) Albumin

Which transporter is responsible for the uptake of fructose from the intestinal lumen into enterocytes?

• (A) GLUT5
• (B) SGLT1
• (C) GLUT2
• (D) Na+/K+-ATPase

What is the primary function of the myenteric (Auerbach's) plexus located between the longitudinal and circular muscle layers of the GI tract wall?

• (A) Control of gastrointestinal motility
• (B) Regulation of secretion and blood flow
• (C) Sensing luminal contents
• (D) Absorption of nutrients

Which enzyme initiates protein digestion in the stomach?

• (A) Pepsin
• (B) Trypsin
• (C) Chymotrypsin
• (D) Carboxypeptidase

In Hirschsprung disease, there is a congenital absence of ganglion cells (submucosal and myenteric plexuses) in a segment of the distal colon. This results in:

• (A) Functional obstruction due to failure of relaxation of the affected segment
• (B) Excessive peristalsis in the affected segment
• (C) Chronic diarrhea
• (D) Malabsorption of fats

Which hormone, secreted by G cells in the gastric antrum, stimulates gastric acid secretion by parietal cells?

• (A) Gastrin
• (B) Somatostatin
• (C) Secretin
• (D) Vasoactive intestinal peptide (VIP)

Carbohydrate digestion is completed at the brush border of small intestinal enterocytes by which group of enzymes?

• (A) Disaccharidases (lactase, sucrase, maltase)
• (B) Pancreatic amylase
• (C) Salivary amylase
• (D) Pepsin

Which pattern of gastrointestinal motility is primarily responsible for mixing chyme with digestive juices and facilitating absorption in the small intestine?

• (A) Segmentation contractions
• (B) Peristalsis
• (C) Migrating motor complex (MMC)
• (D) Mass movements

What is the primary mechanism for sodium absorption in the jejunum?

• (A) Co-transport with glucose, amino acids, and other nutrients
• (B) Exchange with H+ (NHE)
• (C) Diffusion through ENaC channels
• (D) Coupled NaCl transport

Cholecystokinin (CCK) release from duodenal I cells is primarily stimulated by the presence of:

• (A) Fatty acids and amino acids in the duodenum
• (B) Acidic chyme in the duodenum
• (C) Glucose in the duodenum
• (D) Distension of the duodenum

Which condition is characterized by autoimmune destruction of parietal cells, leading to intrinsic factor deficiency and vitamin B12 malabsorption?

• (A) Pernicious anemia
• (B) Tropical sprue
• (C) Whipple disease
• (D) Celiac disease

What is the role of the migrating motor complex (MMC)?

• (A) 'Housekeeping' function, sweeping undigested material through the GI tract during fasting periods
• (B) Mixing chyme in the small intestine
• (C) Propelling chyme rapidly through the colon
• (D) Initiating defecation

Iron absorption occurs primarily in which segment of the small intestine?

• (A) Duodenum
• (B) Jejunum
• (C) Ileum
• (D) Colon

Which transporter is responsible for pumping H+ ions into the gastric lumen, creating the highly acidic environment of the stomach?

• (A) H+/K+-ATPase (proton pump)
• (B) Na+/K+-ATPase
• (C) Ca2+-ATPase
• (D) H+-ATPase

Zollinger-Ellison syndrome is caused by a gastrin-secreting tumor (gastrinoma), typically located in the pancreas or duodenum. What is the major clinical consequence?

• (A) Excessive gastric acid secretion leading to peptic ulcers and diarrhea
• (B) Achlorhydria (lack of stomach acid)
• (C) Gallstone formation
• (D) Constipation

Which enzyme, secreted by the pancreas in an inactive form, is activated by enterokinase/enteropeptidase located on the duodenal brush border, initiating the activation cascade for other pancreatic proteases?

• (A) Trypsinogen
• (B) Chymotrypsinogen
• (C) Procarboxypeptidase
• (D) Proelastase

Saliva contains enzymes that begin the digestion of which two types of macromolecules?

• (A) Carbohydrates and Fats
• (B) Proteins and Fats
• (C) Carbohydrates and Proteins
• (D) Only Carbohydrates

What is the main function of the submucosal (Meissner's) plexus?

• (A) Regulation of local secretion, absorption, and blood flow
• (B) Control of overall gut motility
• (C) Sensing pain and temperature
• (D) Coordinating mass movements

Which part of the small intestine is the principal site for absorption of bile salts and vitamin B12?

• (A) Terminal ileum
• (B) Duodenum
• (C) Jejunum
• (D) Proximal ileum

Steatorrhea (excess fat in feces) is most likely to result from a deficiency in:

• (A) Pancreatic lipase or bile salts
• (B) Salivary amylase
• (C) Pepsin
• (D) Lactase

Achalasia is a motor disorder of the esophagus characterized by impaired relaxation of the lower esophageal sphincter (LES) and aperistalsis. It is caused by degeneration of inhibitory neurons in which neural plexus?

• (A) Myenteric (Auerbach's) plexus
• (B) Submucosal (Meissner's) plexus
• (C) Vagal dorsal motor nucleus
• (D) Celiac plexus

Water absorption in the gastrointestinal tract occurs primarily via:

• (A) Osmosis, driven by solute absorption
• (B) Active transport via aquaporins
• (C) Co-transport with sodium
• (D) Paracellular diffusion only

Which type of contraction propels feces through the large intestine, occurring only a few times per day, often triggered by meals (gastrocolic reflex)?

• (A) Mass movements
• (B) Segmentation contractions
• (C) Peristalsis
• (D) Haustral churning

Lactose intolerance is caused by a deficiency in which brush border enzyme?

• (A) Lactase
• (B) Sucrase
• (C) Maltase
• (D) Isomaltase

Which cells are responsible for secreting mucus in the small intestine?

• (A) Goblet cells
• (B) Paneth cells
• (C) Enteroendocrine cells
• (D) M cells

Somatostatin, secreted by D cells in the GI tract, generally has what effect on digestive processes?

• (A) Inhibitory
• (B) Stimulatory
• (C) No effect
• (D) Variable depending on location

Vasoactive Intestinal Peptide (VIP) is a neurotransmitter/hormone that causes relaxation of GI smooth muscle, including sphincters, and stimulates intestinal fluid and electrolyte secretion. Tumors secreting VIP (VIPomas) typically cause:

• (A) Watery diarrhea, hypokalemia, and achlorhydria (WDHA syndrome)
• (B) Severe peptic ulcer disease
• (C) Constipation and abdominal pain
• (D) Gallstone formation

Where does the final enzymatic digestion of proteins (into amino acids, dipeptides, tripeptides) occur before absorption?

• (A) Brush border and cytoplasm of enterocytes
• (B) Gastric lumen
• (C) Pancreatic duct
• (D) Duodenal lumen

Which reflex involves relaxation of the ileocecal valve/sphincter in response to food entering the stomach?

• (A) Gastroileal reflex
• (B) Gastrocolic reflex
• (C) Enterogastric reflex
• (D) Intestino-intestinal reflex

Paneth cells, located at the base of the crypts of Lieberkühn in the small intestine, primarily function to:

• (A) Secrete antimicrobial peptides (e.g., defensins, lysozyme)
• (B) Secrete mucus
• (C) Absorb water and electrolytes
• (D) Secrete hormones like CCK

Absorption of most dietary calcium occurs primarily in which part of the small intestine, and is enhanced by which vitamin?

• (A) Duodenum and Jejunum; Vitamin D
• (B) Ileum; Vitamin K
• (C) Colon; Vitamin C
• (D) Stomach; Vitamin B12

The defecation reflex is initiated by:

• (A) Distension of the rectum by feces
• (B) Contraction of the external anal sphincter
• (C) Relaxation of the internal anal sphincter
• (D) Presence of food in the stomach

Which enzyme is responsible for activating pancreatic lipase in the duodenum?

• (A) Bile salts (via colipase displacement)
• (B) Trypsin
• (C) Enterokinase
• (D) Chymotrypsin

Hepatocytes conjugate bilirubin with which substance to increase its water solubility before excretion in bile?

• (A) Glucuronic acid
• (B) Sulfate
• (C) Glycine
• (D) Taurine

What is the primary mechanism for chloride absorption in the ileum and colon?

• (A) Coupled exchange with bicarbonate (Cl-/HCO3- exchanger)
• (B) Co-transport with sodium (NKCC)
• (C) Paracellular diffusion
• (D) Active transport via CFTR channel

Gastric emptying is slowed down by the presence of which substances in the duodenum?

• (A) Fat, acid, and hyperosmolar solutions
• (B) Carbohydrates only
• (C) Proteins only
• (D) Hypo-osmolar solutions

Which layer of the GI tract wall contains the major blood vessels, lymphatic vessels, and the submucosal nerve plexus?

• (A) Submucosa
• (B) Mucosa
• (C) Muscularis externa
• (D) Serosa (or Adventitia)