**Core Concept:**
Chemoprophylaxis refers to the use of antimalarial drugs to prevent or reduce the risk of malaria infection in individuals travelling to or residing in endemic areas. In this case, we are discussing the choice of antimalarial drugs for an Englishman visiting a region with chloroquine and mefloquine-resistant malaria.

Malaria is caused by Plasmodium parasites, with the most common species being Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium knowlesi. The most common malaria species in England are Plasmodium falciparum and Plasmodium vivax, with Plasmodium ovale and Plasmodium malariae being less common. Plasmodium knowlesi is rare in England but more common in Southeast Asia.

**Why the Correct Answer is Right:**
The correct answer is A, chloroquine. Although mefloquine is also an effective antimalarial drug, it is not as effective against chloroquine-resistant Plasmodium falciparum malaria as chloroquine. Chloroquine is a first-line drug for prophylaxis in regions where Chloroquine-resistant Plasmodium falciparum is prevalent. Chloroquine is effective against chloroquine-sensitive Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae infections.

**Why Each Wrong Option is Incorrect:**

B: Mefloquine (Option B) is incorrect for two reasons:
1. Mefloquine is not as effective against chloroquine-resistant Plasmodium falciparum malaria as chloroquine.
2. Mefloquine is contraindicated in patients with a history of seizures or psychiatric disorders, whereas chloroquine is generally safer for these patients.

C: Pyrimethamine (Option C) is incorrect because it is primarily used in combination with sulfadoxine (Fansidar) for prophylaxis in areas of chloroquine-resistant Plasmodium falciparum malaria. It is not a first-line antimalarial drug for chloroquine-resistant Plasmodium falciparum malaria.

D: Artemisinin combination therapies (ACTs) are a type of antimalarial treatment, not prophylaxis. They are used for the treatment of uncomplicated Plasmodium falciparum malaria but are not recommended for chemoprophylaxis due to their limited prophylactic efficacy.

**Clinical Pearl:**
Chloroquine prophylaxis is generally well-tolerated, with fewer side effects compared to mefloquine. Chloroquine is a well-established, effective, and widely available drug for chemoprophylaxis in regions with chloroquine-resistant Plasmodium falciparum malaria. It is essential for English travelers to consider chloroquine as their prophylactic drug of choice.