## Core Concept
Malaria is a serious and sometimes life-threatening disease caused by a parasite that is transmitted to humans through the bite of an infected mosquito. There are several species of Plasmodium that can cause malaria, including *Plasmodium vivax*, *Plasmodium ovale*, *Plasmodium malariae*, and *Plasmodium falciparum*. Each species has distinct characteristics and can cause different levels of severity in the disease.

## Why the Correct Answer is Right
The correct statements concerning malaria are:
- *b) Red cells negative for the Duffy blood group antigen are resistant to Plasmodium vivax*: This is because *Plasmodium vivax* uses the Duffy blood group antigen as a receptor to enter red blood cells. Individuals lacking this antigen (Duffy-negative) are resistant to *P. vivax* infection.
- *c) Renal impairment is a grave prognostic sign in falciparum malaria*: Malaria caused by *P. falciparum* can lead to severe complications, including renal failure, which significantly worsens the prognosis.
- *d) Plasmodium malariae can cause immune-mediated nephropathy*: *P. malariae* infection is associated with the development of nephrotic syndrome due to immune complex deposition in the kidneys.

## Why Each Wrong Option is Incorrect
- **Option A:** Malaria caused by each of the four plasmodial species can relapse after initial illness. This statement is not entirely accurate because while *Plasmodium vivax* and *Plasmodium ovale* can indeed relapse due to hypnozoites (dormant liver stages), *Plasmodium falciparum* and *Plasmodium malariae* do not have this capability. *P. falciparum* and *P. malariae* can recrudesce (re-emerge) due to incomplete treatment but not truly relapse from a dormant stage.

## Clinical Pearl / High-Yield Fact
A key point to remember is that individuals of African descent are often Duffy-negative, which provides them with natural resistance to *Plasmodium vivax* infection. This is a significant factor in the epidemiology of malaria in these populations.

## Correct Answer: B.