**Question:** A young boy presents with failure to thrive. Biochemical analysis of a duodenal aspirate after a meal reveals a deficiency of enteropeptidase (enterokinase). The levels of which one of the following digestive enzymes would be affected?

A. Pancreatic lipase
B. Pancreatic amylase
C. Pepsinogen
D. Chymotrypsinogen

**Correct Answer:**

**Core Concept:** Enteropeptidase (enterokinase) is an enzyme that plays a crucial role in the activation of trypsinogen, a precursor to trypsin, in the small intestine. It is essential for the proper functioning of the digestive system, specifically the breakdown of proteins in the intestinal tract.

**Why the Correct Answer is Right:** In this scenario, the deficiency of enteropeptidase leads to improper activation of trypsinogen, resulting in a cascade of effects on other digestive enzymes. Trypsin is responsible for activating peptides into amino acids, which are then absorbed and utilized by the body. Without enteropeptidase, trypsin cannot be activated, affecting its downstream enzymes:

1. **D. Chymotrypsinogen**: Chymotrypsin is an enzyme responsible for breaking down proteins into smaller peptides. Since trypsin is not activated due to enteropeptidase deficiency, chymotrypsinogen, the precursor of chymotrypsin, will also be affected.
2. **Option D (Chymotrypsinogen):** Chymotrypsin is an enzyme responsible for breaking down proteins into smaller peptides. Since trypsin is not activated due to enteropeptidase deficiency, chymotrypsinogen, the precursor of chymotrypsin, will also be affected.

**Why Each Wrong Option is Incorrect:**

1. **A. Pancreatic lipase**: Pancreatic lipase is an enzyme responsible for the digestion of fats, not proteins. Its deficiency would not be affected by enteropeptidase deficiency.
2. **B. Pancreatic amylase**: Similar to pancreatic lipase, amylase is an enzyme responsible for the digestion of carbohydrates (starch). Its deficiency would not be affected by enteropeptidase deficiency, as they are involved in different aspects of digestion.
3. **C. Pepsin**: Pepsin is primarily involved in the breakdown of proteins in the stomach, not the small intestine where enteropeptidase is active. Deficiency of pepsin would not be affected by enteropeptidase deficiency.

**Core Concept**: Enteropeptidase (also known as chymotrypsinogen activation peptidase) is an enzyme responsible for activating chymotrypsinogen, which is the precursor of chymotrypsin. This process occurs in the small intestine, specifically in the upper part of the small intestine called the "duodenum".

**Why Option C (Pepsin) is Incorrect**: Pepsin is a proteinase enzyme found in the stomach, and it plays a crucial role in the breakdown of