Skiers and American football players are prone to medial collateral ligament (MCL) injuries. The MCL is a major ligament of the knee that resides on the inner (medial) side of the knee. The MCL is connected to the tibia and femur, as well as the meniscus and anterior cruciate ligament, making compound injuries common.

The anterior cruciate ligament resides at the front (anterior) side of the knee, while the posterior cruciate ligament resides to the back (posterior) of the knee. The medial meniscus is a fibrocollagen band that is fused with the MCL.

The skeleton plays a role in musclular contraction in that it serves as an anchor and provides leverage against which muscles can exert force. It also serves as protection and support for soft tissues. Bones also maintain certain ion concentrations. The correct answer choice is the regulation of pH through hydration, which is not a function of the skeletal system.

There are organic (collagen) and inorganic (hydroxyapatite) components of the bone that contribute to its characteristic hardness. The key here is knowing the composition of hydroxypatite, which is primarily constructed of calcium and phosphorus.

Collagen is a protein that allows the bone to be somewhat flexible, so "removal of proteins" is the only correct answer. Other answer choices involve inorganic bone material, which add to hardness of the skeleton, but not to its flexibility.

Compact and spongy bone have similar chemical and structural compositions and are both hard and resistant to compression; however, due to the fact that compact bone is denser, nutrients are delievered through canals called Haversian systems in compact bone; spongy bone is less dense and lacks these specialized canals.

The lymphatic system has a variety of functions in the body. It collects excess interstitial fluid and returns it to the blood (cardiovascular system), it has lymph nodes that screen the lymphatic fluid for pathogens (immune system), and it reroutes fat digestates to the neck veins (digestive system). The lymphatic system does not aid the endocrine system, as hormones travel in the bloodstream.

You either know the anatomy of lymphatic flow, or you don't and you need to review it. All statements are true. The thoracic duct is the major collecting point for lymph from both lower limbs, the intestine, and certainly the left arm and left side of the head and neck. It empties into the junction of the left subclavian vein and the left jugular vein, most commonly. The question did not ask you to argue about whether or not lymph from the right arm and right side of the head and neck rejoined the venous circulation through the thoracic duct or a smaller accessory thoracic duct on the right side, because this is somewhat variable.  

It is important to know that the lymphatic system is an open system, meaning that it drains the interstitial fluid from the body and delivers it to the circulatory system in two different ways. The majority of the lymph drains from the body through the thoracic duct into the vena cava. The thoracic duct is also what drains fats collected in the liver and turned into chylomicrons.

Lymph from the head and right arm drains via the right lymphatic duct back to the vena cava, but this does not constitute the majority of lymph flow.

It is important to know that lymph from the head, neck, and right arm drains via the right lymphatic duct into the vena cava, to be added to the venous circulation. The other important lymphatic duct is the thoracic duct, which drains lymph from the remainder of the body. 

Lymphatic fluid is formed by absorbing the excess plasma that has been filtered from the capillaries. This occurs because more plasma is filtered out of the capillaries than can be reabsorbed back into the capillaries. This leads to a net filtration of plasma into the interstitium. The role of the lymphatic system is to absorb this filtered plasma and return it to circulation via a duct into the right atrium. A failure to collect the extracellular fluids can lead to swelling of the extremities, known as edema.

When the lymphatic vessels collect fluid they also carry it through the lymph nodes, which house large populations of lymphocytes. These lymphocytes screen the blood for foreign antigens and can launch an immune response if pathogens are found.