The key idea is where the genetic material is kept and how the cell is guided. The nucleus is the organelle that houses the cell’s DNA and acts as the command center, controlling which genes are turned on or off and directing overall cellular activities. It’s enclosed by a double membrane called the nuclear envelope, with channels (pores) that regulate what enters and leaves. Inside, DNA is organized with proteins as chromatin, and the nucleolus within the nucleus is where ribosomal RNA is made to build ribosomes. Mitochondria do carry some DNA, but their main role is energy production, not directing the cell. The Golgi apparatus processes and ships proteins, and ribosomes synthesize proteins, but neither stores the genetic blueprint. So the organelle that houses DNA and steers the cell’s activities is the nucleus.

The lymphatic system is responsible for returning excess interstitial fluid to the circulatory system and supporting immune function. Interstitial fluid, which bathes tissues, is mostly reabsorbed by blood capillaries, but a portion enters lymphatic capillaries as lymph. This lymph travels through progressively larger lymphatic vessels, is filtered by lymph nodes where immune cells can detect and respond to pathogens, and then returns to the bloodstream, emptying into the subclavian veins via the thoracic duct (and the right lymphatic duct on the right). Lymphoid organs like the spleen and thymus coordinate immune responses, while lymph carries immune cells and antigens to monitor for invaders. The lymphatic system also helps absorb fats from the digestive tract through lacteals.

An initial alert to invasion comes from cells that release chemical signals to kickstart inflammation. Basophils are white blood cells that release histamine and other mediators when tissue is invaded. Histamine increases blood vessel permeability and promotes inflammation, helping bring in more immune cells to the site and signaling that there’s a threat. That signaling role is why basophils are described as alerting the body to invasion. Lymphocytes are involved in a later, targeted immune response; erythrocytes carry oxygen; platelets are mainly for clotting and wound repair rather than signaling an invasion.

Gonarthrosis is the degenerative arthritis of the knee, i.e., osteoarthritis of the knee. It involves wear and tear of the joint cartilage, narrowing of the joint space, and changes in the bones, which over time can cause chronic pain, stiffness, and reduced function. When the disease becomes severe and other treatments don’t relieve symptoms, knee replacement is often considered to restore function and alleviate pain. Bursitis would present as swelling and tenderness from inflamed knee bursa rather than a primary degenerative joint issue. Osgood-Schlatter disease is an overuse injury at the tibial tubercle typically seen in adolescents. Chondromalacia refers to cartilage softening under the kneecap and causes anterior knee pain, but the term used for long-standing, joint-wide degenerative knee disease that may require replacement is gonarthrosis.

The main concept here is understanding the pathway of systemic circulation. The aorta is the largest artery because it is the primary vessel that leaves the heart to distribute blood to the entire body. Right after the left ventricle contracts, blood enters the aorta, which has the greatest diameter and carries the highest volume of blood as it feeds the arterial branches that reach every organ and tissue. Its thick, elastic walls are built to withstand the strong pressure generated with each heartbeat and to help propel blood forward through the arterial system. The other options aren’t the answer for this reason: the vena cava is a vein, not an artery, so it carries blood back to the heart. The pulmonary artery is indeed an artery, but it carries blood to the lungs and is smaller in diameter than the aorta. The coronary arteries are small arteries that supply the heart muscle itself, not the entire body.