Is it established that Muhsin was the fifth child of Ali from Fatimah?

Answer

In the name of Allah, there has never been, nor is there, any doubt among the Shia regarding this. Yes, some scholars may, out of precaution (taqiyya), refrain from openly affirming the matter of Muhsin, as seems to be the case in some statements of Sheikh al-Mufid. Also, know that Muhsin, the martyred grandson (through oppression by the oppressor), is considered the fifth child only if we say that Ali ibn Abi Talib (PBUH) had two daughters from Fatimah (peace be upon her), namely Zaynab and Umm Kulthum. However, this is a matter of scholarly dispute, as it is strongly possible that Zainab is the one who was also known as Umm Kulthum. Allah is the Guide and He is the All-Knowing.