✅Answer: First, pay attention to the following narrations:

1 عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ وَ بِإِسْنَادِهِ عَنْ مُحَمَّدِ بْنِ عَلِيِّ بْنِ مَحْبُوبٍ عَنْ يَعْقُوبَ بْنِ يَزِيدَ وَ بِإِسْنَادِهِ عَنْ عَلِيِّ بْنِ مَهْزِيَارَ جَمِيعاً عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ حَمَّادِ بْنِ عُثْمَانَ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ سَمِعْتُ أَبَا جَعْفَرٍ يَقُولُ لَا يَضُرُّ الصَّائِمَ مَا صَنَعَ إِذَا اجْتَنَبَ ثَلَاثَ [اربع] خِصَالٍ الطَّعَامَ وَ الشَّرَابَ وَ النِّسَاءَ وَ الِارْتِمَاسَ فِي الْمَاءِ.

📚 Al-Tahdheeb, vol. 4, p. 189

Muhammad ibn Muslim said: 'I heard Imam Baqir, peace be upon him, saying: 'Provided that a person who is fasting stays away from three (four) things, what he does will not harm him: eating, drinking, women, and submerging oneself in water.'

2 عَنْ مُحَمَّدِ بْنِ الْحَسَنِ عَنِ الصَّفَّارِ عَنْ أَحْمَدَ بْنِ أَبِي عَبْدِ اللَّهِ عَنْ أَبِيهِ رَفَعَهُ إِلَى أَبِي عَبْدِ اللَّهِ قَالَ: خَمْسَةُ أَشْيَاءَ تُفَطِّرُ الصَّائِمَ الْأَكْلُ وَ الشُّرْبُ وَ الْجِمَاعُ وَ الِارْتِمَاسُ فِي الْمَاءِ وَ الْكَذِبُ عَلَى اللَّهِ وَ عَلَى رَسُولِهِ وَ عَلَى الْأَئِمَّةِ ع.

📚 Al-Khisal, p. 286, hadith 39

Imam Sadiq, peace be upon him, said: 'Five things break and invalidate the fast: eating, drinking, sexual intercourse, submerging oneself in water, and intentionally lying about God, the Messenger, and the Ahl al-Bayt, peace be upon them.'

✅ The most important things that invalidate the fast are mentioned in the two narrations above. Although there are other cases mentioned in other narrations that are annexed to these, and there are also cases that are avoided during fasting based on the principle of caution, the point is that the things that invalidate the fast are limited, and nothing but the cases mentioned in the narrations invalidates the fast. A nosebleed is not among the things that the Ahl al-Bayt, peace be upon them, mentioned as invalidating the fast.

✅ Now, if a person has a nosebleed and some blood unintentionally enters his throat, pay attention to the following two narrations:

3 عَنْ عَلِيِّ بْنِ مَهْزِيَارَ أَنَّهُ سَأَلَهُ يَعْنِي أَبَا الْحَسَنِ الثَّالِثَ... إلی أن قال.... وَ كُلُّ مَا غَلَبَ اللَّهُ عَلَيْهِ فَاللَّهُ أَوْلَى بِالْعُذْرِ.

📚 Al-Faqih, vol. 1, p. 363

Abul Hasan al-Thalith, i.e., Imam Ali ibn Muhammad al-Naqi (the tenth Imam), peace be upon him, said in a hadith: 'Whatever Allah has overpowered a servant with, then Allah is more deserving of accepting the excuse.'

4 عَنْ مُوسَى بْنِ بَكْرٍ قَالَ: قُلْتُ لِأَبِي عَبْدِ اللَّهِ الرَّجُلُ يُغْمَى عَلَيْهِ يَوْماً أَوْ يَوْمَيْنِ أَوِ الثَّلَاثَةَ أَوِ الْأَرْبَعَةَ أَوْ أَكْثَرَ مِنْ ذَلِكَ كَمْ يَقْضِي مِنْ صَلَاتِهِ قَالَ أَ لَا أُخْبِرُكَ بِمَا يَجْمَعُ لَكَ هَذا و أَشْبَاهِهِ، كُلُّ مَا غَلَبَ اللَّهُ عَلَيْهِ مِنْ أَمْرٍ فَاللَّهُ أَعْذَرُ لِعَبْدِهِ.

قَالَ وَ زَادَ فِيهِ غَيْرُهُ أَنَّ أَبَا عَبْدِ اللَّهِ قَالَ: هَذَا مِنَ الْأَبْوَابِ الَّتِي يُفَتِّحُ كُلُّ بَابٍ مِنْهَا أَلْفَ بَابٍ.

📚 Al-Khisal, p. 644, hadith 24

Musa ibn Bakr said: 'I said to Imam Sadiq, peace be upon him: 'A person becomes unconscious for one day, two days, three days, four days, or more; how many of his prayers should he make up?' The Imam said: 'Shall I not inform you of something that combines this case and its likes for you!'

'Whatever Allah has overpowered a person with in a matter, then Allah is more accepting of the excuse for His servant!'

And another narrator in this hadith added that Imam Sadiq, peace be upon him, said: 'This (principle) is from the chapters that each chapter opens a thousand chapters!'

✅ According to the above important and widely used principle, if a person has a nosebleed and blood unintentionally enters his throat, the Almighty and Exalted Lord accepts his excuse, and there is no problem.

✅ Learning the above principle and applying it in the right context clarifies the answers to many of our questions.