A. Ring M. falciparum: This is the most likely diagnosis given the patient's symptoms and recent travel history to Africa. Malaria caused by Plasmodium falciparum is the most severe form and can lead to anemia, splenomegaly, and cerebral malaria, which could explain the headache. The presence of ring forms in the blood smear is characteristic of this infection.

B. P. vivax: While less severe than P. falciparum, P. vivax can still cause significant morbidity, including anemia and splenomegaly. It is also prevalent in various parts of the world, including Africa.
C. P. ovale: Similar to P. vivax, P. ovale is another form of malaria that can cause the patient's symptoms. It is less common than P. falciparum and P. vivax but should be considered in the differential diagnosis.

D. P. malariae: Although less common, P. malariae can cause a more chronic form of malaria, leading to anemia and splenomegaly. It is crucial not to miss this diagnosis due to its potential for long-term morbidity if left untreated.
Severe bacterial infections (e.g., typhoid fever, brucellosis): These infections can present with similar symptoms, including fever, headache, and splenomegaly. Missing these diagnoses could lead to severe consequences, including death, if not promptly treated with appropriate antibiotics.

Babesiosis: This is a rare parasitic infection that can cause anemia and other symptoms similar to malaria. It is typically transmitted by ticks and is more common in certain parts of the world, but it should be considered in travelers to endemic areas.
Visceral leishmaniasis: This parasitic infection can cause splenomegaly, anemia, and fever. Although it is rare, it is an important consideration in travelers to areas where the disease is endemic, such as parts of Africa and Asia.