Pneumonia: This is the most likely diagnosis given the symptoms. Increased interstitial markings can indicate an infection or inflammation in the lung tissue, and patch opacity in one lung area is consistent with a localized infection such as pneumonia.

Pulmonary Edema: This condition, characterized by fluid accumulation in the lungs, can cause increased interstitial markings and patchy opacities, especially if it's due to heart failure or high altitude.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: An exacerbation of COPD can lead to increased interstitial markings due to inflammation and patchy opacities from bronchitis or bronchiolitis.
Interstitial Lung Disease (ILD): Conditions like idiopathic pulmonary fibrosis can present with increased interstitial markings, though patchy opacities might be less common.

Pulmonary Embolism: Although less likely to cause patchy opacities, a pulmonary embolism can lead to areas of lung infarction that might appear as patchy opacities, and it's crucial not to miss this diagnosis due to its high mortality rate.
Tuberculosis: Especially in endemic areas, tuberculosis can present with a variety of lung findings, including increased interstitial markings and patchy opacities, and is critical not to overlook due to its public health implications.
Lung Cancer: Metastatic disease or primary lung cancer can cause patchy opacities, and while less likely, missing this diagnosis could have significant implications for patient outcomes.

Lymphangitic Carcinomatosis: A rare condition where cancer spreads to the lymphatic vessels in the lungs, causing interstitial markings and potentially patchy opacities.
Eosinophilic Pneumonia: A rare condition characterized by an accumulation of eosinophils in the lungs, which can cause increased interstitial markings and patchy opacities.
Sarcoidosis: Although more commonly associated with hilar lymphadenopathy, sarcoidosis can also cause interstitial lung disease and patchy opacities in some cases.