The organ in the human female that develops to nourish the embryo is called

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: 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.

Correct Answer: C

Rationale: Smooth muscle lacks striations banded patterns from aligned sarcomeres unlike striated skeletal (multinucleate, voluntary) and cardiac (uninucleate, involuntary, branched) muscles. Involuntary control and single nuclei are shared with cardiac muscle, and branching is cardiac-specific. Smooth muscle's non-striated, spindle-shaped cells suit its role in visceral organs, distinguishing its microscopic appearance and function from the organized contractile bands of other types, key for histological identification.

Correct Answer: B

Rationale: A fascicle, a bundle of muscle fibres (cells), is larger than a single fibre, which contains myofibrils bundles of sarcomeres. Sarcomeres, the smallest contractile units, are within myofibrils. Fascicles, wrapped in perimysium, contribute to whole-muscle structure, distinguishing them as the largest listed unit, key to muscle organization and force production.