**Core Concept**
The question pertains to the development and maturation of various bodily structures. At birth, certain physiological and anatomical features are not fully developed or present, whereas others are already mature. This concept is crucial in understanding pediatric and neonatal medicine.

**Why the Correct Answer is Right**
The correct answer is related to the development of the pancreas and its ability to produce digestive enzymes. At birth, the pancreas is not fully mature and lacks the ability to produce sufficient digestive enzymes, which are essential for protein digestion. This is because the pancreas develops and matures postnatally, and it takes several weeks for the pancreas to produce adequate amounts of digestive enzymes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the liver is present at birth and is already functional, producing bile and participating in detoxification processes.
**Option B:** This option is incorrect because the kidneys are present at birth and are already functioning, filtering waste products from the blood.
**Option C:** This option is incorrect because the stomach is present at birth and is already producing gastric acid and digestive enzymes.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that neonates are unable to digest lactose due to the immaturity of the pancreas and the lack of lactase production. This is why breastfed infants often develop lactose intolerance in the first few months of life.

**Correct Answer: C. Stomach**