A 4-year-old boy is brought to the physician because of frequent respiratory tract infections and chronic diarrhea. His stools are bulky and greasy, and he has around 8 bowel movements daily. He is at the 10th percentile for height and 25th percentile for weight. Chest examination shows intercostal retractions along with diffuse wheezing and expiratory rhonchi. Which of the following is the most likely cause of his condition?

Intracellular retention of misfolded proteins

Increased frequency of trinucleotide repeats

Altered configuration of a protease inhibitor

Defective ciliary protein function

Frameshift mutation of muscle-anchoring proteins

A 3-year-old boy is brought to the physician because he is easily fatigued and has not gained weight. He eats 3 meals and has 3 to 4 bowel movements daily with bulky stools that float. He had recurrent episodes of sinusitis in infancy. He is at the 15th percentile for height and 5th percentile for weight. Examination shows pale conjunctivae. A few scattered expiratory crackles are heard in the thorax. There is abdominal distention. Which of the following is the most likely underlying cause of this patient's failure to thrive?

Exocrine pancreatic insufficiency

Impaired intestinal amino acid transport

Mucosal damage from excessive gastric acid

Intestinal inflammatory reaction to gluten

Small intestine bacterial overgrowth

A scientist is studying the mechanism by which the gastrointestinal system coordinates the process of food digestion. Specifically, she is interested in how distension of the lower esophagus by a bolus of food changes responses in the downstream segments of the digestive system. She observes that there is a resulting relaxation and opening of the lower esophageal (cardiac) sphincter after the introduction of a food bolus. She also observes a simultaneous relaxation of the orad stomach during this time. Which of the following substances is most likely involved in the process being observed here?

Vasoactive intestinal polypeptide

A 23-year-old man presents to the office complaining of weight loss and fatigue for the past 2 months. He states that he has been experiencing foul-smelling, light-colored stools but thinks it is because he hasn’t been eating well, recently. He has a past medical history significant for cystic fibrosis, which is well-controlled medically. He denies any shortness of breath, chest or abdominal pain, nausea, vomiting, or melena. On physical examination, his skin is pale and dry. Which of the following would be the most likely etiology of a malabsorption syndrome giving rise to this patient’s current condition?

Decreased recycling of bile acids

Autoimmune damage to parietal cells

Chronic damage to intestinal mucosa

Damage to intestinal brush border

A medical student is studying digestive enzymes at the brush border of the duodenum. He isolates and inactivates an enzyme in the brush border that has a high affinity for the pancreatic proenzyme trypsinogen. When the enzyme is inactivated, trypsinogen is no longer converted to its active form. Which of the following is the most likely underlying mechanism of this enzyme?

Cleavage of a propeptide from an N-terminus

Attachment of a carbohydrate to a side chain

Phosphorylation of an amino acid side chain

Carboxylation of a glutamate residue

Conjugation of ubiquitin to lysine residue

A 4-year-old girl is brought to the physician by her parents because she is severely underweight. She is easily fatigued and has difficulty keeping up with other children at her daycare. She has a good appetite and eats 3 full meals a day. She has 4 to 5 bowel movements daily with bulky, foul-smelling stools that float. She has had recurrent episodes of sinusitis since infancy. Her parents report that she recently started to snore during her sleep. She is at the 15th percentile for height and 3rd percentile for weight. Her vital signs are within normal limits. Examination shows pale conjunctivae. A few scattered expiratory crackles are heard in the thorax. There is abdominal distention. Which of the following is the most likely underlying cause of this patient's failure to thrive?

Exocrine pancreatic insufficiency

T. whippelii infiltration of intestinal villi

Impaired intestinal amino acid transport

Small intestine bacterial overgrowth

Intestinal inflammatory reaction to gluten

A 39-year-old woman presents to her primary care physician because she has been experiencing intermittent abdominal pain for the last 2 weeks. She says that the pain is squeezing in nature, is located in the right upper quadrant, and is particularly severe after eating a meal. After a diagnosis is made, the patient asks why the pain gets worse after eating. The physician explains that food is detected by the gastrointestinal tract through numerous receptors and that this information is transmitted to other parts of the body to cause compensatory changes. The neurons responsible for transmitting this information are most likely located in a layer of the intestine that has which of the following characteristics?

Contracts to generate peristaltic waves

Contracts to generate local movement in mucosa

Contains cells that primarily absorb nutrients

Connective tissue that envelops the other layers

Contains large blood vessels and large lymphatic vessels

Protein digestion and absorption for USMLE Step 2 CK: High-Yield Physiology Lessons

Gastric Phase - The Acid Test

Stimulus: Vagal input & stomach distension trigger gastrin release.
HCl Secretion: Parietal cells secrete HCl, lowering gastric pH to ~1.5-3.5.
Pepsinogen Activation: Chief cells release inactive pepsinogen.

⭐ Pepsin is an endopeptidase that preferentially cleaves peptide bonds after aromatic amino acids (e.g., Phenylalanine, Tyrosine).

Gastric Chief and Parietal Cells Histology

Intestinal Lumen - Pancreatic Power-up

Chyme entering the duodenum triggers the release of pancreatic zymogens (inactive proenzymes) to prevent pancreatic autodigestion. Their activation is a critical, cascaded event.

Primary Activation: Brush border enzyme enteropeptidase (enterokinase) converts trypsinogen to its active form, trypsin.
Cascade: Trypsin then activates other pancreatic zymogens.

Endopeptidases: Trypsin, chymotrypsin, elastase cleave internal peptide bonds.
Exopeptidases: Carboxypeptidases A & B cleave amino acids from the C-terminus.
Result: Large proteins are broken down into smaller, absorbable oligopeptides, dipeptides, and tripeptides.

⭐ High-Yield Fact: Enteropeptidase, a brush border enzyme, is the master switch for activating protein digestion in the intestine. Its deficiency leads to severe protein malabsorption and failure to thrive.

Brush Border & Absorption - The Final Gateway

Final Digestion: Occurs at the apical membrane of enterocytes. Brush border enzymes hydrolyze small peptides into absorbable units.
- Enzymes: Aminopeptidases and dipeptidases cleave oligopeptides into amino acids, dipeptides, and tripeptides.
Absorption into Enterocyte:
- Amino Acids: Absorbed via sodium-dependent co-transporters (e.g., B⁰AT1 for neutral AAs).
- Di- & Tripeptides: Absorbed faster via a proton-dependent co-transporter, PepT1.
- Inside the cell, most di/tripeptides are hydrolyzed to amino acids by cytoplasmic peptidases.
Exit to Blood: Amino acids exit the enterocyte via facilitated diffusion across the basolateral membrane into the portal circulation.

⭐ Hartnup Disease: An autosomal recessive defect in the B⁰AT1 transporter for neutral amino acids (e.g., Tryptophan). Leads to pellagra-like symptoms (dermatitis, dementia, diarrhea) due to deficient niacin synthesis.

Protein Digestion & Absorption in Enterocyte

Clinical Correlates - Pathway Pathologies

Pancreatic Insufficiency
- Causes: Chronic pancreatitis, cystic fibrosis, pancreatic cancer.
- Mechanism: ↓ synthesis & secretion of pancreatic proteases (trypsin, chymotrypsin).
- Result: Protein malabsorption, azotorrhea (excess fecal nitrogen), and edema.
Congenital Enteropeptidase Deficiency
- Rare autosomal recessive disorder preventing trypsinogen activation.
- Leads to severe, generalized protein maldigestion.
- Presents in infancy with failure to thrive, diarrhea, and hypoalbuminemia-induced edema.
Amino Acid Transport Defects
- Hartnup Disease: Defective neutral amino acid (tryptophan) transporter in intestinal/renal cells.
  
  ⭐ Causes pellagra-like symptoms (dermatitis, dementia, diarrhea) due to impaired niacin (B3) synthesis from tryptophan.
- Cystinuria: Defective dibasic amino acid transporter (Cysteine, Ornithine, Lysine, Arginine 📌 COLA).
  - Intestinal defect is clinically silent; main issue is recurrent cystine kidney stones from renal transport failure.

High‑Yield Points - ⚡ Biggest Takeaways

Protein digestion begins in the stomach with pepsin; most occurs in the small intestine.

Pancreatic proteases (trypsin, chymotrypsin) are secreted as inactive zymogens.

Brush border enterokinase activates trypsinogen to trypsin, initiating an activation cascade.

Final products are amino acids, dipeptides, and tripeptides, absorbed by specific transporters.

Na+-dependent cotransport is the primary mechanism for amino acid absorption.

Hartnup disease is a key pathology involving defective neutral amino acid transport.

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