The bursae pad the gaps between bones, muscles, tendons, and skin. The bursae, a singular bursa, act as a kind of bubble cover for the body’s structures. Bursitis develops when a bursa swells and becomes inflamed.

Actinic keratosis (AK) is a skin condition that results in rough, scaly skin areas. Solar keratosis is another term for AK. Since actinic keratosis is a form of precancer, it can develop into cancer (squamous cell carcinoma) if left untreated.

Acanthamoeba—a free-living almost-invisible to-eye amoeba—is responsible for spreading rare infections affecting the eye, skin, and central nervous system. The pathogen is found in water and soil around the world. The ameba can spread through contact lens use, skin wounds, and cuts or can reach the lungs through inhalation.

Arrhythmias, also known as abnormal heart rhythms, occurs when the electrical impulses that control the heart’s pumping do not function properly. Irregular heart rhythms can occur in the upper or lower chambers of the heart, causing the heart to beat too fast, too slowly, or irregularly.

Achalasia is a rare and chronic disorder that affects the ability of the esophagus to move food to the stomach. The esophagus is a muscular tube that carries food and fluids from the throat to the stomach.

Stretch marks are a common phenomenon in men and women. The marks are a form of scarring on the skin. They can appear on any part of the body but are most commonly found on the skin around the belly, lower back, breasts, thighs, hips, and upper arms.

Achondroplasia is a genetic disorder that affects bone growth and development. A mutation in the FGFR3 gene, which regulates bone growth, causes this form of dwarfism. A person only needs to inherit one copy of the mutated gene from one parent to develop the condition.

Small, benign growths known as acrochordons usually have the same color as the skin epidermis. They frequently resemble a mass of skin tissue growing from a small stem. They can occasionally be darker and mimic a raised mole.