The main idea is how emphysema destroys the lung’s acinar units. The acinus, starting at the respiratory bronchiole and including the alveolar ducts and sacs, can be damaged in different patterns. In panacinar emphysema, the destruction is uniform across the entire acinus—every part is involved. This pattern is classically linked to alpha-1 antitrypsin deficiency and often affects the lower lungs. In contrast, centriacinar emphysema targets mainly the proximal part of the acinus around the respiratory bronchioles, with distal alveolar parts relatively spared. That’s typically seen with smoking. Bronchiectasis isn’t an emphysema pattern at all; it’s dilation of the bronchi from chronic infection and wall damage, a different process. Lower lobe emphysema describes location rather than a destruction pattern of the acinus, and while panacinar can be lower-lobe predominant, the defining feature here is the uniform involvement of the whole acinus. So the description that fits all parts of the acinus being damaged is panacinar emphysema.

Functional residual capacity is the volume in the lungs after a normal exhale, and it increases when air is trapped in the lungs at the end of expiration. In obstructive diseases, the airways are narrowed and expiratory flow is limited, so during expiration not all the air can be pushed out. This leads to air trapping and a higher end-expiratory lung volume, which raises the functional residual capacity. In contrast, restrictive diseases make the lungs stiffer and smaller, so the end-expiratory volume is reduced, not increased. Normal lungs sit at a baseline FRC, and mixed disease can vary but the characteristic rise due to air trapping points to obstructive pathology.

Residual volume is the air left in the lungs after a full forced exhalation. In restrictive disease, the lungs and chest wall are stiff and cannot expand well, causing a reduction in overall lung capacity. Because the lungs are smaller and airways aren’t allowed to empty into a larger capacity, the amount of air left after exhalation (the residual volume) tends to be reduced. In obstructive disease, airway narrowing leads to air trapping, so residual volume increases. Mixed disease often follows the obstructive pattern with higher residual volume as well. Normal would have the usual, unchanged residual volume.

Charcot-Leyden crystals come from eosinophils. They are made from galectin-10, a major protein stored in eosinophil granules. When eosinophils are activated and release their granule contents during allergic or parasitic inflammation, galectin-10 can crystallize and form the characteristic crystals seen in sputum or tissue. This association with eosinophilic inflammation explains why these crystals appear in conditions like asthma and parasitic infections. Neutrophils, platelets, or lymphocytes do not produce these crystals, so their granule contents don’t form Charcot-Leyden crystals.