## **Core Concept**
Tubocurarine is a non-depolarizing neuromuscular blocking agent that competes with acetylcholine for receptors at the neuromuscular junction, leading to muscle relaxation. It affects different muscle groups at varying rates due to differences in muscle physiology and receptor distribution.

## **Why the Correct Answer is Right**
The correct answer, **D. Small muscles of the face and neck**, is based on the fact that tubocurarine, like other non-depolarizing neuromuscular blockers, tends to relax muscles in a specific sequence. This sequence generally starts with the small muscles of the face and neck, followed by the muscles of the limbs, and finally the trunk muscles. The small muscles of the face and neck are relaxed first because they have a higher concentration of nicotinic acetylcholine receptors and are more sensitive to the effects of neuromuscular blocking agents.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the muscles of the abdominal wall are not among the first to be relaxed by tubocurarine; they are relaxed later in the sequence.
- **Option B:** This option is incorrect because, although the muscles of the limbs are affected relatively early, they are not the first to be relaxed by tubocurarine.
- **Option C:** This option is incorrect because the diaphragm, being a vital muscle for respiration, is actually one of the last muscles to be affected by neuromuscular blockers like tubocurarine, due to its critical function and different muscle fiber composition.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that non-depolarizing neuromuscular blockers like tubocurarine cause muscle relaxation in a specific sequence: small muscles of the face and neck, then limbs, and finally the trunk and diaphragm. This sequence is crucial for understanding the clinical effects and for managing patients under neuromuscular blockade.

## **Correct Answer:** . Small muscles of the face and neck