Primaquine acts on which stage of life cycle of all human malarial parasites for causal prophylaxis?
**Core Concept**
Primaquine is an antimalarial drug that targets the dormant forms of certain human malarial parasites, specifically for the purpose of causal prophylaxis. This involves the destruction of hypnozoites, which are the dormant stages of Plasmodium vivax and Plasmodium ovale, to prevent relapse of the disease.
**Why the Correct Answer is Right**
The mechanism of action of primaquine involves the activation of oxidative stress within the parasite, leading to the destruction of hypnozoites. This is achieved through the reduction of 6-methoxy-8-demethyl-8-hydroxyquinine (6-MDMP) to 6-hydroxy-8-hydroxyquinine (6-OH-8-OH), which then reacts with glutathione to form a reactive intermediate that damages the parasite's DNA. This process is specific to the hypnozoite stage of P. vivax and P. ovale, making primaquine an effective agent for causal prophylaxis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because primaquine does not act on the blood stages of malaria parasites, which are the stages that cause symptoms and are targeted by other antimalarial drugs such as chloroquine and artemisinin.
**Option B:** This option is incorrect because primaquine does not act on the erythrocytic stage of malaria parasites, which is the stage that infects red blood cells and causes symptoms.
**Option C:** This option is incorrect because primaquine does not act on the sporozoite stage of malaria parasites, which is the stage that infects the liver and is targeted by other antimalarial drugs such as tafenoquine.
**Clinical Pearl / High-Yield Fact**
Primaquine is the only antimalarial drug that targets the hypnozoite stage of P. vivax and P. ovale, making it essential for the prevention of relapse in these infections. It is also important to note that primaquine can cause hemolysis in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency, making it essential to screen patients for this condition before administering the drug.
**Correct Answer: D.**