The muscle that forms the major portion of the cheek is the:

Correct Answer: D

Rationale: The buccinator is the primary muscle of the cheek, forming its muscular foundation and aiding in chewing by compressing the cheek against the teeth. Located between the maxilla and mandible, it's essential for keeping food in the oral cavity. The masseter, a powerful jaw-closing muscle, is superficial to the cheek but not its main component. The temporalis elevates the mandible from above the cheek. The zygomaticus major lifts the mouth corners for smiling, not forming the cheek. The buccinator's role and position make it the correct answer, as it directly constitutes the cheek's structure, distinguishing it from other facial muscles involved in jaw or expression movements.

Correct Answer: A

Rationale: During inspiration, external intercostals act as synergists to the diaphragm, elevating the ribs to expand the thoracic cavity while the diaphragm flattens, increasing volume for air intake. External obliques and rectus abdominis compress the abdomen, aiding exhalation, not inspiration. Internal intercostals depress ribs in forced exhalation, opposing inspiration. External intercostals are the correct answer, as they assist the diaphragm's primary role in breathing, enhancing chest expansion, a critical synergy in respiratory mechanics.

Correct Answer: B

Rationale: Muscles are specialized tissues responsible for movement and are classified based on structure and function. The three distinct types are cardiac, smooth, and skeletal. Cardiac muscle powers the heart, contracting involuntarily to pump blood. Smooth muscle lines organs like the stomach and blood vessels, also working involuntarily to manage processes like digestion. Skeletal muscle, attached to bones, enables voluntary movements like walking or lifting. Tendons, ligaments, and joints are connective structures, not muscle types tendons link muscle to bone, ligaments connect bones, and joints are bone junctions. Flexion and extension describe movements, not muscle categories, and stringy isn't a scientific term for muscle. The correct classification reflects the histological and functional diversity of muscle tissue, critical for understanding their roles in the body, distinguishing them from structural or movement descriptors that don't define muscle itself.

Correct Answer: A

Rationale: Anaerobic respiration occurs without oxygen, breaking down glucose for energy in cells, producing lactic acid instead of carbon dioxide and water. This happens during intense exercise when oxygen demand exceeds supply, like sprinting. Aerobic respiration requires oxygen, yielding more energy efficiently in mitochondria. Mastication is chewing, unrelated to respiration. 'None of the above' dismisses the clear process. Anaerobic respiration's oxygen-free nature suits short, high-energy bursts, contrasting aerobic's sustained effort, and its lactic acid byproduct explains muscle fatigue, distinguishing it from unrelated terms or processes, aligning with cellular energy dynamics under specific conditions.

Correct Answer: D

Rationale: Muscles controlling substance flow within lumens like blood vessels, digestive tract, or airways are smooth muscles. These involuntary muscles, found in organ walls, contract or relax to regulate movement, such as peristalsis in the gut or vasoconstriction in arteries, without conscious control. The hormonal system involves chemical signaling, not muscle action. The skeletal system includes voluntary muscles for locomotion, not lumen regulation. Cardiac muscles power the heart, a specific function unrelated to general lumen control. Smooth muscles' unique ability to modulate flow across diverse tubular structures makes them the best fit, as their autonomic regulation and adaptability distinguish them from striated muscles in managing internal transport.