Regarding carbohydrate metabolism:

Correct Answer: D

Rationale: Correct Answer: D - The conversion of pyruvate to acetyl-CoA is irreversible because it is catalyzed by the enzyme pyruvate dehydrogenase. This step is a key regulatory point in carbohydrate metabolism as it commits pyruvate to enter the citric acid cycle for further energy production. Reversing this step would disrupt the normal flow of metabolic pathways. Summary of other choices: A: Glucokinase is not increased in starvation; it regulates glucose uptake in the liver. B: The breakdown of glycogen is called glycogenolysis, not glycolysis which is the breakdown of glucose. C: The direct oxidative pathway involves the breakdown of glucose through glycolysis, not triose.

Correct Answer: C

Rationale: The correct answer is C: neural. In this scenario, the student experiences the fight or flight response, which is controlled by the sympathetic nervous system. The pupil dilation, increased heart rate, and rapid breathing are all physiological responses triggered by the activation of neural pathways in response to a perceived threat or stress. The other choices are incorrect because: A: Humoral stimuli involve changes in blood levels of certain substances triggering hormone release. B: Hormonal stimuli involve the release of hormones in response to other hormones. D: Positive feedback is a mechanism that amplifies a response, not directly related to the immediate physiological responses seen in this case.

Correct Answer: C

Rationale: The correct answer is C: Oesophagus. The oesophagus is the structure leading to the stomach that carries food from the mouth to the stomach through peristalsis. The pharynx (A) is the throat cavity, the epiglottis (B) is a flap that prevents food from entering the trachea during swallowing, and the larynx (D) is the voice box. Therefore, the oesophagus is the correct answer as it specifically functions to transport food to the stomach.

Correct Answer: D

Rationale: The correct answer is D. Hormones travel through the bloodstream while neurotransmitters travel across the synaptic cleft between neurons. Hormones are secreted by endocrine glands into the bloodstream to target distant tissues, while neurotransmitters are released by neurons into the synaptic cleft to transmit signals locally between neurons. Choice A is incorrect as hormones do not travel through the synaptic cleft, and neurotransmitters do not travel through the bloodstream. Choice B is incorrect as both hormones and neurotransmitters act on specific receptors. Choice C is incorrect as hormones do not use electrical impulses; instead, they travel through the bloodstream.

Correct Answer: A

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.