Atelectasis is the collapse or incomplete expansion of lung tissue that leads to reduced oxygenation. When alveoli aren’t ventilated, a ventilation-perfusion mismatch occurs, causing lower oxygen levels. It also predisposes to infection because collapsed airways harbor stagnant secretions, impair clearance, and create a favorable environment for bacteria. This process is typically reversible when the underlying cause—such as an airway obstruction or external compression—is relieved and the lung re-expands. Resorption atelectasis is one mechanism of this process, while contraction atelectasis from fibrotic scarring is usually less reversible, and pulmonary edema causes hypoxemia through fluid in the alveoli rather than true collapse of alveoli.

This presentation is classic for carcinoid syndrome, caused by a neuroendocrine tumor that releases serotonin and other vasoactive substances into the circulation. The episodic flushing with accompanying diarrhea arises from serotonin-driven increases in gut motility and vasodilation. Cyanosis can occur during these episodes as the vasomotor changes and hypoxemia intensify. A key point is that these substances reach the systemic circulation when liver metastases are present or when substances bypass hepatic metabolism, allowing widespread effects. Over time, continued secretion can also lead to right-sided valvular heart disease from fibrosis. Other options don’t fit this pattern. Horner’s syndrome features ptosis, miosis, and anhidrosis, not diarrhea or flushing. SVC syndrome presents with facial and upper-body edema and dilated chest veins due to venous obstruction, not the secretory diarrhea and flushing. Central tumors is too broad and lacks the characteristic hormonal symptom cluster. Diagnosis is supported by elevated urinary 5-HIAA and imaging with somatostatin receptor–positive scans; management includes somatostatin analog therapy and tumor-directed treatments.

Microvascular pulmonary edema occurs when the capillary endothelium becomes leaky, so fluid escapes into the interstitium and alveoli due to increased permeability rather than because of high hydrostatic pressure. Inflammation from pneumonia triggers this permeability change, with cytokines and inflammatory cells injuring the capillary walls and promoting protein-rich fluid leakage into the air spaces. That inflammatory, permeability-driven mechanism fits microvascular edema best, making pneumonia a consistent cause. Elevated pulmonary pressures point to edema from increased hydrostatic pressure (cardiogenic edema, such as left heart failure), not the microvascular permeability process. Volume overload similarly increases hydrostatic pressure and leads to cardiogenic-type edema.

The key idea is that pulmonary artery pressure rises when the resistance to blood flow in the pulmonary circulation increases. Pulmonary pressure is shaped by how much blood the heart sends into the lungs and how hard the pulmonary vessels resist that flow. If the pulmonary vascular resistance goes up, the same amount of blood flow must push through stiffer or narrower vessels, leading to higher pressure in the pulmonary artery. Other factors like blood viscosity have some effect but are not the primary driver of a marked rise in pulmonary artery pressure. A slower heart rate lowers cardiac output, which tends to reduce the amount of blood entering the pulmonary circulation not increase its pressure. Decreased cardiac output can affect pressures elsewhere, but the direct cause of a higher pulmonary arterial pressure is increased resistance in the pulmonary vasculature.