**Core Concept**
Chloroquine is an antimalarial medication with immunomodulatory and anti-inflammatory properties, making it useful in treating various autoimmune and infectious diseases. Its mechanism of action involves lysosomotropic effects, interfering with the functioning of lysosomes, and anti-inflammatory effects through the inhibition of phospholipase A2 activity.

**Why the Correct Answer is Right**
Chloroquine is effective in treating discoid lupus erythematous (DLE) due to its ability to inhibit the immune system's attack on the skin, reducing inflammation and preventing scarring. In rheumatoid arthritis (RA), chloroquine helps by reducing inflammation and suppressing the immune system's attack on the joints. Additionally, chloroquine is used to treat infectious mononucleosis (IM) caused by Epstein-Barr virus (EBV) due to its antiviral and anti-inflammatory properties, which help alleviate symptoms and prevent complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Discoid lupus erythematous is one condition where chloroquine is effective, but it is not the only condition listed, making this option incomplete.
**Option B:** Rheumatoid arthritis is another condition where chloroquine is useful, but like option A, it is not the only condition listed.
**Option C:** Infectious mononucleosis is a condition where chloroquine is used, but it is not the only condition listed, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Chloroquine's antimalarial properties are due to its ability to accumulate in acidic lysosomes and disrupt the food vacuole membrane, ultimately leading to the death of the malaria parasite. This mechanism of action is also responsible for its immunomodulatory and anti-inflammatory effects in autoimmune diseases.

**✓ Correct Answer: D. All of the above**