**Question:** Dales vasomotor re-reversal is due to

*A. Inhibition of histamine release from mast cells*
*B. Inhibition of calcium influx into smooth muscle cells*
*C. Inhibition of the conversion of L-arginine to L-citrulline by nitric oxide synthase*
*D. Inhibition of the release of norepinephrine from sympathetic nerve endings*

**Correct Answer:** D. Inhibition of the release of norepinephrine from sympathetic nerve endings

**Core Concept:** Dales vasomotor re-reversal is a phenomenon where the vasoconstriction induced by a vasoconstrictor substance is reversed by another substance. This can be explained through the understanding of the autonomic nervous system and the role of sympathetic and parasympathetic nerves in maintaining blood vessel tone.

**Why the Correct Answer is Right:** Vasomotor tone is maintained by a balance between the actions of sympathetic vasoconstrictor neurotransmitters (e.g., norepinephrine) and parasympathetic vasodilator neurotransmitters (e.g., acetylcholine). Dales vasomotor re-reversal results from the inhibition of norepinephrine release from sympathetic nerve endings. This leads to a decrease in the vasoconstrictor effect and subsequent reversal of the vasoconstriction induced by the initial vasoconstrictor substance.

**Why Each Wrong Option is Incorrect:**

A. Inhibition of histamine release from mast cells does not directly affect the vasomotor tone, as histamine is a vasodilator and does not contribute to vasoconstriction reversal.

B. Inhibition of calcium influx into smooth muscle cells would not result in vasomotor re-reversal, as this relates to the smooth muscle contractility rather than the neurotransmitter release from sympathetic nerve endings.

C. Inhibition of nitric oxide synthase, which converts L-arginine to L-citrulline, does not directly affect vasoconstrictor neurotransmitter release from sympathetic nerve endings, making it an incorrect answer for Dales vasomotor re-reversal.

**Clinical Pearl:** Dales vasomotor re-reversal is a fundamental concept in understanding the autonomic nervous system and the role of sympathetic and parasympathetic nerves in maintaining blood vessel tone. This knowledge is essential for clinical decision-making in managing patients with altered blood vessel tone or blood pressure fluctuations. Understanding vasomotor re-reversal can assist in identifying the cause of such changes and guide appropriate therapeutic interventions.