Unilateral leg redness, swelling, warmth, and pain may indicate which of the following?

A. Third-degree (full-thickness) is incorrect because third-degree burns extend through the entire epidermis and dermis and may involve underlying tissues such as fat, muscle, or bone. These burns are typically painless due to nerve destruction and often appear white, leathery, or charred. Blisters are usually absent because the dermis is destroyed.

B. Second-degree (partial thickness) is incorrect because second-degree burns involve the epidermis and part of the dermis. They are characterized by redness, blistering, and severe pain. Partial-thickness burns may be superficial or deep depending on how much of the dermis is affected.

C. Fourth-degree (full-thickness) is incorrect because fourth-degree burns extend beyond the skin into underlying tissues, including muscle, tendon, or bone. These burns are severe, often painless due to nerve destruction, and require surgical intervention.

D. First-degree (superficial) is correct because first-degree burns affect only the epidermis. They cause redness, mild swelling, and pain, but blisters are absent. Common causes include mild sunburn or brief contact with hot objects. First-degree burns typically heal within 3–7 days without scarring, as only the outermost layer of skin is damaged.

A. Larynx is incorrect because the larynx functions mainly as a passageway for air between the pharynx and trachea and houses the vocal cords. It does not participate in gas exchange.

B. Trachea is incorrect because the trachea serves as a conduit for air to move to the bronchi and lungs. Its rigid cartilage rings keep the airway open, but it does not have structures for oxygen and carbon dioxide diffusion.

C. Alveoli is correct because alveoli are tiny, thin-walled sacs at the end of the bronchioles where oxygen diffuses from inhaled air into the blood and carbon dioxide diffuses from the blood into the alveoli to be exhaled. The alveolar walls are extremely thin and surrounded by a dense network of capillaries, maximizing surface area for efficient gas exchange, which is the primary function of the respiratory system.

D. Bronchi is incorrect because the bronchi are larger airways that conduct air into each lung and then divide into smaller bronchioles. While they transport air deeper into the lungs, gas exchange does not occur in the bronchi.