**Core Concept**
The question tests the pharmacological principle of antithyroid drugs, specifically their mechanism of action and onset of effect. Antithyroid drugs are used to treat hyperthyroidism by inhibiting thyroid hormone production or release. Iodides, in particular, rapidly decrease thyroid hormone synthesis and release.

**Why the Correct Answer is Right**
Iodides, such as sodium iodide (NaI) and potassium iodide (KI), work by rapidly inhibiting the release of pre-formed thyroid hormones from the thyroid gland. This is achieved through the Wolff-Chaikoff effect, where excessive iodine uptake by the thyroid gland temporarily blocks the organization of iodine into thyroid hormones. Additionally, iodides inhibit the synthesis of new thyroid hormones by reducing the activity of thyroid peroxidase, an enzyme essential for thyroid hormone production. This results in a rapid decrease in circulating thyroid hormone levels, making iodides the fastest-acting antithyroid medications.

**Why Each Wrong Option is Incorrect**
**Option B:** Propylthiouracil is a thionamide antithyroid drug that inhibits thyroid hormone synthesis by blocking the enzyme thyroid peroxidase. While effective, its onset of action is slower compared to iodides.

**Option C:** Methimazole is another thionamide antithyroid drug that works similarly to propylthiouracil. Its onset of action is also slower than that of iodides.

**Option D:** Nitrates are not used as antithyroid medications. They are primarily used to treat angina pectoris by reducing myocardial oxygen demand.

**Clinical Pearl / High-Yield Fact**
Iodides are often used as a temporary measure to rapidly decrease thyroid hormone levels before initiating long-term antithyroid medication therapy. They are also used as a diagnostic tool to assess the functional status of the thyroid gland.

**✓ Correct Answer: A. Iodides of Na/ K**