The component parts of the myenteric plexus of the enteric nervous system include the following except:

Correct Answer: D

Rationale: The correct answer is D: Control centers in the medulla oblongata. The myenteric plexus is a network of neurons located in the gastrointestinal tract, regulating gut motility independently. The myenteric plexus consists of sensory neurons, interneurons, and motor neurons. Control centers in the medulla oblongata are part of the central nervous system and are not directly involved in the myenteric plexus's functions. Therefore, they are not considered as component parts of the myenteric plexus.

Correct Answer: D

Rationale: The correct answer is D: Intrinsic factor. Saliva does not normally contain intrinsic factor, which is a protein produced by the stomach to aid in the absorption of vitamin B12. Saliva does contain isozymes, IgA antibodies, and lactoferrin. Isozymes are different forms of enzymes, IgA is an antibody important for immune defense, and lactoferrin is a protein with antimicrobial properties found in saliva. Therefore, D is the correct answer as intrinsic factor is not a normal component of saliva.

Correct Answer: B

Rationale: The correct answer is B: Hirschsprung's disease. The absence of ganglion cells in the distal colon is a key diagnostic feature of Hirschsprung's disease. Ganglion cells are essential for coordinated contractions of the colon. In congenital pyloric stenosis, the issue lies in the pylorus, not the colon, so ganglion cells are not relevant. Choice C is incorrect because congenital pyloric stenosis does not involve ganglion cells. Choice D is also incorrect as Hirschsprung's disease does indeed involve the absence of ganglion cells.

Correct Answer: C

Rationale: The correct answer is C because carcinoma of the esophagus often spreads locally, making complete removal difficult. This local spread can involve nearby structures, leading to a poor prognosis. Metastases to the liver or lung (choice A) typically occur later in the disease course, not before diagnosis. While some esophageal tumors may be challenging to access surgically (choice B), this alone does not explain the poor prognosis. Esophageal cancer can respond to radiotherapy (choice D), so radioresistance is not the primary reason for the poor prognosis. In summary, the local spread preventing complete removal is the key factor contributing to the poor prognosis in esophageal carcinoma.

Correct Answer: B

Rationale: The correct answer is B because the buccal phase of swallowing is a voluntary process where the tongue moves the bolus of food towards the back of the mouth to initiate swallowing. This phase is under conscious control and initiates the swallowing reflex. Choice A is incorrect because the buccal phase is voluntary, not involuntary. Choice C is incorrect because the bolus is moved towards the back of the mouth, not rolled over the front of the tongue. Choice D is incorrect because the movement of food from the pharynx to the esophagus occurs during the pharyngeal phase of swallowing, not the buccal phase.