A 26-year-old man presents with jaundice and dark urine after taking primaquine for malaria prophylaxis. What is the most likely diagnosis based on his presentation?

The symptoms described in the scenario, including jaundice and dark urine after taking primaquine, strongly indicate a hemolytic reaction due to glucose-6-phosphate dehydrogenase (G6PD) deficiency. Primaquine is known to induce oxidative stress in individuals with G6PD deficiency, leading to the destruction of red blood cells and resulting in hemolytic anemia.

In patients with G6PD deficiency, certain medications, infections, or foods can trigger hemolysis when they produce oxidative compounds that the body cannot effectively neutralize, given the enzyme's critical role in the pentose phosphate pathway and protection against oxidative damage. The resultant hemolytic anemia manifests as jaundice due to increased bilirubin levels from the breakdown of hemoglobin, as well as dark urine, which reflects the presence of bilirubin in the urine.

The other conditions listed, such as sickle cell anemia, pernicious anemia, and thalassemia, are characterized by different pathophysiological mechanisms that do not predominantly feature an acute hemolytic process triggered by oxidative stress in response to a medication like primaquine. Thus, G6PD deficiency is the most fitting diagnosis in this case.