Sutra:
74. akAraththAlum makAraththAlum rakshakaniyum rakshyaththaiyum solliRRu; chathurththiyAlum ukAraththAlum, rakshaNahEthuvAna prApthiyaiyum palaththaiyum solliRRu.

Meaning:
The akAram spoke of the Protector; the makAram spoke of that which is protected; the fourth case spoke of that which is the reason for the protection; and the ukAram spoke of the result.

Sri PBA Swami's Sarartha Deepikai:
In the 72nd and 73rd sutras, the combined meaning of the prAnavam was shown in one way. There the sEshatvam that is in the hidden fourth case inside the akAram was kept in focus. Now, the root of the akAram is used to show another combined meaning for the praNavam.

The root for akAram is taken as ava-rakshaNE and as the meaning of that root is protection, that is kept in focus and used to derive the meaning of the praNavam here.

The akAram shows Him who is the protector. The makAram shows that which is to be protected which is the Atma. The reason that He protects is because the Atma is enslaved to Him; therefore the fourth case speaks of the sEshatvam which is the reason for the protection. The result of this protection is that the Atma continues in the service of the same protector and this fact that the Atma is undeviatingly enslaved only to Him is spoken by the ukAram.

Thus, the praNavam speaks of the rakshaka, the rakshaNeeya, the rakshaNa hEtu and the rakshana prayOjana.