**Core Concept**
The question is testing the student's understanding of the concept of acid-base chemistry in relation to the excretion of weakly basic drugs. Weak bases, such as the antihistaminic drug mentioned, can be excreted more efficiently by the kidneys when the urine is acidic, as this increases the degree of ionization of the drug in the urine.

**Why the Correct Answer is Right**
The correct answer, A, is that the administration of NH4Cl, an acidifying agent, would accelerate urinary excretion of the antihistaminic drug. This is because the acidic environment of the urine increases the degree of ionization of the weak base, making it more water-soluble and more readily excretable by the kidneys. The ionization of the drug can be represented by the equation: RNH2 ⇌ RNH3+ + e-, where RNH2 is the weak base and RNH3+ is its conjugate acid. In an acidic environment, the equilibrium shifts to the right, increasing the concentration of the ionized form of the drug.

**Why Each Wrong Option is Incorrect**
* **Option B:** The administration of NaHCO3, an alkalinizing agent, would actually decrease the degree of ionization of the weak base, making it less soluble in water and less readily excretable by the kidneys.
* **Option C:** The degree of ionization of the drug is not affected by the pH of the stomach, but rather by the pH of the urine. The stomach pH is not relevant to the excretion of the drug.
* **Option D:** The absorption of the drug is not affected by the pH of the stomach, but rather by the solubility of the drug in the gastrointestinal tract. The weak base would be more soluble in the acidic environment of the stomach, but this does not affect its absorption.

**Clinical Pearl / High-Yield Fact**
In cases of poisoning with weakly basic drugs, the administration of an acidifying agent, such as NH4Cl, can increase the degree of ionization of the drug in the urine, making it more readily excretable by the kidneys. This can be an important consideration in the management of such cases.

**✓ Correct Answer: A. Urinary excretion would be accelerated by administration of NH4Cl, an acidifying agent**