Okay, let's tackle this question. The question is about how long immunity lasts after a full course of tetanus toxoid with three doses. Hmm, I need to remember the immunization schedules.

First, the core concept here is active immunity against tetanus. Tetanus toxoid is an inactivated toxin that stimulates the immune system to produce antibodies. The primary series is three doses, right? Then there are boosters.

Now, the correct answer is probably around 5-10 years, but I'm not sure. Let me think. The initial three doses are given at 2, 4, and 6 months in some schedules, but the question is about the duration of immunity after completing the series. Wait, after the primary series, the immunity is not lifelong. You need boosters every 10 years. But maybe after the full course, the protection lasts 5-7 years before a booster is needed. Wait, I'm getting confused. Some sources say that after the primary series, immunity is considered to last for about 7-10 years. Then, boosters are recommended every 10 years. But the question is about the initial immunity after three doses. So maybe the answer is 5-10 years, but the exact number? Let me check my notes. Oh right, the primary series provides protection for about 5-7 years, and then a booster every 10 years is needed. So if the options include 10 years, that's incorrect because the initial immunity isn't that long. So if the correct answer is 5 years, that's the one. But I need to confirm. The options aren't given, but the correct answer is likely 5 years. Let me structure the explanation accordingly.

Now, the wrong options: maybe options like 3 years or 15 years. For example, if an option says 3 years, that's too short. If one says 10 years, that's the duration after a booster, not the initial series. Another might be 1 year, which is way too short. The correct answer should be 5-7 years, so the options might have 5 years as the correct choice. I need to make sure the explanation is accurate and covers the mechanism, why boosters are needed, and the clinical pearl about the schedule.

**Core Concept** Tetanus toxoid vaccination induces active immunity by stimulating antibody production against the tetanus toxin. The primary series (3 doses) establishes long-term protection, but antibody levels wane over time, necessitating periodic boosters.

**Why the Correct Answer is Right** The full primary series of tetanus toxoid (typically given at 2, 4, and 6 months in infants or as a 0-, 1-, and 6-month schedule in adults) provides protective antibody levels for **5–7 years**. After this period, antibody titers decline significantly, increasing the risk of tetanus if exposed. This duration reflects the balance between immune memory and the need for booster doses to maintain immunity.

**Why Each Wrong Option is Incorrect**
**Option A:** (e.g., "3 years") Incorrect. This underestimates the duration of protection; the primary series provides longer immunity than boosters (which last ~10 years).
**Option B:** (e.g., "10 years") Incorrect. A 1

✓ Correct Answer: B. 10