Correct Answer: A

Rationale: Gallstones become most problematic when they block a duct, such as the bile duct or cystic duct. This blockage can lead to various complications, including inflammation, infection, and even damage to the gallbladder or liver. In severe cases, a blocked duct can cause intense pain, jaundice, and other serious symptoms that require medical intervention. Choices B, C, and D are incorrect because the presence of cholesterol or bilirubin in gallstones or the process of dissolution do not necessarily indicate when gallstones become most problematic. The critical issue arises when the stones obstruct a duct, leading to complications.

Correct Answer: C

Rationale: The correct answer is C: frontal. The coronal plane of the body is also known as the frontal plane. This plane divides the body into front (anterior) and back (posterior) sections. It is perpendicular to the sagittal plane and divides the body into front and back portions. The term 'coronal' is often used in radiology and anatomy to describe this specific anatomical plane. Choices A, B, and D are incorrect because the sagittal plane divides the body into left and right portions, the median plane is another name for the sagittal plane, and the axial plane is a term less commonly used to describe an anatomical plane.

Correct Answer: D

Rationale: The integumentary system, which includes the skin, hair, and nails, is primarily responsible for maintaining temperature control in the body. The skin helps regulate body temperature through processes like sweating to cool down and shivering to generate heat. Additionally, blood vessels in the skin can dilate or constrict to control the amount of heat lost or retained by the body. This system plays a crucial role in maintaining homeostasis and keeping the body at a stable temperature. The skeletal system (Choice A) provides structural support and protection to the body but is not directly involved in temperature regulation. The digestive system (Choice B) is responsible for breaking down food and absorbing nutrients, not for regulating body temperature. The nervous system (Choice C) controls body functions through electrical impulses but is not the primary system responsible for temperature control.

Correct Answer: D

Rationale: Melatonin is the hormone produced by the pineal gland. It plays a crucial role in regulating the sleep-wake cycle and is instrumental in maintaining the body's circadian rhythm. Melatonin production is triggered by darkness and inhibited by light, aiding in signaling the body when it is time to sleep. Corticosteroid, oxytocin, and prolactin are hormones produced by other glands in the body and are not associated with the pineal gland. Therefore, they are incorrect choices for this question.

Correct Answer: A

Rationale: The frontal lobe of the brain, specifically the left hemisphere in an area known as Broca's area, is responsible for controlling speech production. This region plays a vital role in speech formation and language expression. Damage to the frontal lobe, particularly Broca's area, can lead to language difficulties like expressive aphasia, a condition characterized by trouble speaking fluently. The parietal lobe is more involved in processing sensory information and spatial awareness. The temporal lobe is associated with memory, hearing, and language comprehension. The occipital lobe is primarily responsible for processing visual information.