The liberation of proteins to free amino acids (final digestive step) occur in all of the following EXCEPT:

Correct Answer: D

Rationale: The liberation of proteins to free amino acids occurs primarily in the small intestine lumen. In the stomach, proteins are broken down into smaller peptides but not fully liberated to amino acids. The enterocyte cytoplasm is where amino acids are absorbed. The brush border aids in absorption but doesn't release proteins to amino acids. Therefore, the correct answer is D (stomach) as it is not the site for the final step of protein digestion.

Correct Answer: B

Rationale: The correct answer is B: vagus. The efferent (motor) swallow reflex involves motor nerves that control swallowing muscles. The vagus nerve (cranial nerve X) is primarily responsible for the afferent (sensory) component of the swallow reflex, while the efferent component is carried out by other cranial nerves such as the glossopharyngeal (IX), facial (VII), and hypoglossal (XII) nerves. Therefore, the vagus nerve is not directly involved in the efferent motor response during swallowing. Trigeminal (V), facial (VII), and hypoglossal (XII) nerves play essential roles in the motor aspects of swallowing, making them incorrect choices.

Correct Answer: C

Rationale: Rationale for Correct Answer (C): 1,25(OH)2 D3 and PTH stimulate both osteoblasts and osteoclasts. 1,25(OH)2 D3 enhances calcium absorption in the gut, promotes bone resorption, and stimulates osteoclast activity. PTH increases calcium levels in the blood by enhancing calcium reabsorption in the kidney and stimulating osteoclast activity. Both hormones indirectly stimulate osteoblasts by increasing calcium availability for bone formation. Summary of Incorrect Choices: A: Osteoclasts actually secrete acid, not alkaline phosphatase. B: Osteoblasts are derived from mesenchymal stem cells, not monocytes. D: Estrogens have a dual effect, inhibiting bone resorption by osteoclasts and stimulating bone formation by osteoblasts; their main effect is inhibiting osteoclast activity, not solely stimulating osteoblasts.

Correct Answer: C

Rationale: The correct answer is C because 1,25-dihydroxycholecalciferol, the active form of vitamin D, does induce calcium binding protein synthesis in mucosal cells, facilitating calcium absorption. This process occurs in the small intestine. Choices A and B are incorrect because around 30% of calcium is typically absorbed, and active transport of Ca2+ primarily occurs in the upper small intestine. Choice D is incorrect because phosphate does not directly facilitate calcium absorption.

Correct Answer: A

Rationale: The correct answer is A: islets of Langerhans. These clusters of cells in the pancreas are responsible for producing hormones such as insulin and glucagon. They play a crucial role in regulating blood sugar levels. The other choices, B: pancreatic medulla, C: pancreatic cortex, and D: nodules, are incorrect as they do not accurately describe the specific clusters of cells in the pancreas responsible for hormone production. It is important to understand the function and location of the islets of Langerhans to grasp the endocrine role of the pancreas.