The 'Sinoatrial (SA) node' is known as the natural pacemaker of the heart because: MCQ with Answer and Explanation

Explanation:
Prolactin, secreted by the anterior pituitary, is the primary hormone responsible for milk synthesis (lactation) after childbirth. Oxytocin causes milk ejection (let-down reflex). Estrogen and progesterone stimulate mammary duct and alveolar development during pregnancy but inhibit milk secretion; after delivery, their levels drop, allowing prolactin to act.

Explanation:
The skin and mucous membranes form the first line of defense (innate immunity), acting as physical barriers that prevent pathogens from entering the body. White blood cells (leukocytes), including phagocytes and lymphocytes, are the cellular components of the immune system that identify, attack, and destroy pathogens. Antibiotics are drugs designed to kill or inhibit the growth of bacteria; they are completely ineffective against viruses, which lack the cellular machinery targeted by antibiotics. Thus, statements 1 and 2 are correct.

Explanation:
Estrogen promotes ductal growth in breasts and deposition of fat, contributing to breast development at puberty. Progesterone develops alveoli, prolactin synthesizes milk, oxytocin ejects milk.