The peculiar and intense symptom picture described – obstinate constipation for years, a sensation of stricture in the rectum, and agonizing pain during bowel movements even with the aid of an enema, likened to labor pains – is a characteristic symptom strongly associated with Syphilinum in homeopathic materia medica. This remedy, derived from the syphilitic miasm, often presents with deep-seated, chronic, and intensely painful conditions. The feeling of constriction or stricture in orifices and the severity of the pain are key indicators for Syphilinum.
Let's briefly consider the other options:
\begin{itemize}
\item Sulphur: Known for various skin and constitutional symptoms, and constipation can be a part of its picture, but not typically with this intense, stricture-like sensation and agonizing pain.
\item Nux Vomica: Indicated for ineffectual urging to stool, often with a feeling as if not finished, and irritability, but not typically the sensation of stricture and labor-like pain.
\item Nitric Acid: Associated with splinter-like pains and fissures, often in the rectum, but the overall picture described is more characteristic of Syphilinum.
\end{itemize}
Therefore, based on the unique combination and intensity of the described rectal symptoms, Syphilinum is the most likely remedy.