Pleural effusion is a common abnormality detected in additional tests in patients hospitalized in Internal Medicine Departments. Properly carried out diagnostics allows to determine the cause of the fluid, thanks to which it is possible to initiate appropriate treatment. The paper presents the case of a 63-year-old man admitted to a district hospital due to dyspnea on exertion. Chest X-ray revealed fluid in the right pleura. The patient was admitted to the Department of Internal Medicine for diagnostics. Due to the large amount of fluid in the pleural cavity, the patient required multiple punctures. The article discusses the most important diagnostic tests that should be performed to determine the cause of the presence of fluid. A key role was assigned to distinguishing the etiology of the fluid based on Light’s criteria, and other features of the fluid enabling its differential diagnosis were taken into account. Possible diagnostic points were also presented and an attempt was made to briefly discuss the correct procedure. During the diagnostics, pleural lymphoma was suspected and the patient was referred to a higher-reference center for further diagnostics. The author presents the necessary diagnostic steps necessary to establish the final diagnosis. The article is presented in the form of single-choice questions in which the reader can actively participate. The reading was enriched with imaging studies.