What are the plasma-soluble substances that are secreted by gram-positive bacteria called?

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.

Correct Answer: D

Rationale: During pregnancy, the placenta forms from maternal and fetal tissues, connecting the embryo to the uterine wall. It transfers oxygen and nutrients from the mother's blood to the fetus's, removes waste, and produces hormones like progesterone to sustain pregnancy. The amnion encases the fetus in fluid, the yolk sac provides early nutrients in other species, and the fallopian tube transports eggs. The placenta's nourishing role, critical for fetal development, distinguishes it as the key gestational organ in humans.

Correct Answer: A

Rationale: Somatic motor (efferent) neurons, part of the voluntary nervous system, release acetylcholine at neuromuscular junctions to initiate skeletal muscle contraction, driving movements like lifting. Sympathetic neurons regulate involuntary functions (e.g., heart rate), not skeletal muscle. Sensory neurons transmit stimuli to the CNS, not activating muscles. 'Muscle neuron' isn't a term neurons innervate, not reside in, muscle. Somatic motor neurons' direct, voluntary control distinguishes them, essential for skeletal muscle's deliberate action, unlike autonomic or sensory roles.

Correct Answer: B

Rationale: Contraction of many sarcomeres, via actin-myosin sliding, shortens the myofibril, a bundle of sarcomeres within a fiber, reducing overall muscle length. Thick filaments (myosin) don't shorten actin slides over them. Motor proteins (myosin) drive this, not shortening themselves. Sarcoplasmic reticulum releases Ca²⁺, not contracting. Myofibril shortening aggregates sarcomere action, distinguishing it, key to muscle movement.

Correct Answer: A

Rationale: Muscle fibres contain nested structures. Myosin, a protein molecule, forms thick myofilaments bundles of myosin within sarcomeres, the contractile units. Sarcomeres align in myofibrils, spanning the fibre. Myosin, at the molecular level (about 160 nm long), is smaller than myofilaments (micrometres), sarcomeres (2 micrometres), and myofibrils (cell-length). Its role as a building block for thicker structures marks it as the smallest, foundational to contraction mechanics, distinguishing it from larger assemblies in muscle hierarchy.