**Core Concept**
The diagnosis of HIV infection in infants born to HIV-infected mothers is crucial for early intervention and treatment. However, the immune system of infants is immature, and the body's ability to produce antibodies in response to HIV infection is delayed. This makes it challenging to diagnose HIV infection in infants using antibody-based tests like ELISA.

**Why the Correct Answer is Right**
The ELISA test relies on the detection of HIV antibodies in the blood. Infants born to HIV-infected mothers can have HIV antibodies present in their blood due to maternal transmission, but these antibodies can also be present due to the mother's immune response. The risk of false-positive results is high in the first two years of life. The Centers for Disease Control and Prevention (CDC) recommend that ELISA testing for HIV infection in infants be repeated at 18 months of age to confirm the diagnosis. This is because, by 18 months, most infants with HIV infection will have developed their own antibodies, and the risk of false-positive results due to maternal antibodies decreases significantly.

**Why Each Wrong Option is Incorrect**
**Option B:** 9 months - At 9 months, the risk of false-positive results due to maternal antibodies is still high, and the ELISA test may not be reliable for diagnosing HIV infection.
**Option C:** 12 months - While 12 months is closer to the recommended age, the risk of false-positive results is still significant, and the test may not be reliable for diagnosing HIV infection.
**Option D:** 3 months - At 3 months, the risk of false-positive results due to maternal antibodies is extremely high, and the ELISA test is not reliable for diagnosing HIV infection in this age group.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the diagnosis of HIV infection in infants requires a combination of clinical evaluation, laboratory testing, and follow-up. The CDC recommends that all infants born to HIV-infected mothers receive HIV testing at birth, 1-2 months, 4-6 months, and 9-12 months, with a confirmatory test at 18 months.

✓ Correct Answer: A. 18 months