What can cause hyponatremia?

Choice A reason: Facial trauma may increase aspiration risk if swallowing is impaired, but a 21-year-old typically has a robust immune system, reducing pneumonia risk. The trauma does not directly compromise lung defenses or immune function significantly, making this individual less likely to develop pneumonia compared to an immunocompromised person.

Choice B reason: Osteoarthritis and chronic knee pain in a 72-year-old may limit mobility, slightly increasing pneumonia risk due to reduced lung expansion. However, this condition does not directly impair immune function or lung clearance mechanisms, making it a lower risk compared to immunodeficiency, which severely predisposes to infections.

Choice C reason: Elevated cholesterol in a 48-year-old increases cardiovascular risk but does not directly affect lung function or immune response to infections. Without respiratory or immune compromise, this individual has a lower risk of pneumonia compared to someone with a primary immunodeficiency disorder, making this choice incorrect.

Choice D reason: A 69-year-old with a primary immunodeficiency disorder has a severely compromised immune system, impairing the ability to fight respiratory infections like pneumonia. Age-related lung changes and reduced immune response increase susceptibility to pathogens, making this individual at the highest risk for developing pneumonia, which is why this is correct.

Choice A reason: Hemoglobin does not store oxygen in the lungs; it binds oxygen in the alveoli and releases it in tissues. Oxygen storage is minimal, and myoglobin in muscles serves this role, making this choice incorrect for hemoglobin’s primary function in gas exchange.

Choice B reason: Hemoglobin does not prevent carbon dioxide from entering the bloodstream. It binds carbon dioxide in tissues and transports it to the lungs for exhalation, facilitating gas exchange, not blocking it, making this choice incorrect for hemoglobin’s role.

Choice C reason: Hemoglobin binds oxygen in the lungs, forming oxyhemoglobin, and transports it via red blood cells to body tissues, where it releases oxygen for cellular metabolism. This is its primary role in gas exchange, making this the correct choice.

Choice D reason: Hemoglobin does not produce oxygen; oxygen is inhaled from the environment. Hemoglobin’s role is to transport oxygen, not synthesize it, making this choice incorrect as it misrepresents the physiological function of hemoglobin in gas exchange.