What shortens the sarcomere during muscle contraction?

Correct Answer: B

Rationale: Sarcomere shortening, the basis of muscle contraction, occurs via the sliding filament mechanism. Myosin heads, protruding from thick filaments, bind to actin on thin filaments, forming cross-bridges. Using ATP-derived energy, these heads execute a power stroke, pulling actin toward the sarcomere's center specifically the A band's midpoint reducing the distance between Z lines and shortening the sarcomere. Acetylcholine initiates this by triggering an action potential, but it's not the direct shortening mechanism. Calcium release from the sarcoplasmic reticulum enables myosin-actin binding by shifting tropomyosin, a crucial step but not the act of shortening. ATP breakdown powers the process, yet the sarcoplasmic reticulum stores calcium, not hydrolyzes ATP for shortening. The myosin-actin interaction is the mechanical driver, making it the precise answer, central to muscle movement and the sliding filament theory.

Correct Answer: D

Rationale: Elite athletes in prolonged high-intensity exercise (>2.5 hours), like marathons, should consume 90 g of glucose plus fructose per hour. This mix leverages multiple transporters (SGLT1 for glucose, GLUT5 for fructose), maximizing absorption beyond glucose's 60 g/h limit, delivering ~1.5 g/min of energy. Solo 60 g glucose caps at 1 g/min, insufficient for sustained high intensity. Adding fructose to 60 g boosts uptake slightly, but 90 g optimizes fuel, reducing fatigue. Excess glucose alone (90 g) overloads SGLT1, risking GI distress. The dual-carb approach distinguishes it, critical for elite endurance performance.

Correct Answer: C

Rationale: The temporalis muscle spans the side of the head, originating from the temporal bone's fossa and inserting on the mandible, elevating it for chewing. The forehead hosts frontalis for brow movement. The neck has muscles like sternocleidomastoid for head turning. The chin area features mentalis for lip motion. Temporalis' lateral cranial position and masticatory role distinguish it, essential for jaw closure, unlike forehead, neck, or chin functions.

Correct Answer: B

Rationale: Triceps brachii extends the forearm, straightening the elbow, with its three heads inserting on the ulna. Biceps brachii flexes it. Brachialis flexes beneath biceps. Deltoid abducts the arm, not elbow-focused. Triceps' extension role distinguishes it, essential for pushing, unlike flexors or shoulder movers.

Correct Answer: B

Rationale: Gram-positive bacteria, with thick peptidoglycan walls, secrete potent, soluble proteins known as exotoxins into the bloodstream. These toxins, like tetanus or botulinum toxins, are released during bacterial growth, targeting specific host cells and causing diseases such as diphtheria or food poisoning. Endotoxins, conversely, are lipopolysaccharides from gram-negative bacteria's outer membranes, released upon cell lysis, not secreted. The generic term 'toxin' lacks specificity, and 'none of the above' dismisses the clear distinction. Exotoxins' solubility in plasma and their secretion mechanism highlight their role in pathogenesis, contrasting with endotoxins' structural origin, making them the precise answer for gram-positive bacterial products.