In “The Enemy” we see Worf put in a position to save the injured Romulan as he is the only member of the crew to have the ribosomes needed to save his life. He is initially reluctant, but gives in, only for the Romulan to insult and reject Worf as a donor and he dies later on.
You would think that with Dr. Selar on board and the similarity of Vulcan and Romulan physiology, that she would be able to supply the ribosomes needed no problem. Was she away at a conference or something, making her unavailable, as I don't remember that being mentioned in the episode at all?
In short: Plot contrivance for one-sided narrative.
All Vulcans on the ship were tested. Keep in mind that Vulcans and Romulans share the most similar genetic material bar none. Yet a Klingon, vastly different in structure, was by far the most compatible donor... the way the episode frames the dilemma doesn't allow for what some later Trek (DS9, et al) have added regarding medical help in various areas... But Dr Crusher was not talking about medical tinkering - just naturally occurring plotdevices needed via a simple blood transfusion.
Ribosomes translate genetic code into chains of amino acids - they are protein builders for cells...
Fun article:
https://sciencetrends.com/the-function-of-ribosomes/
Human ribosomes and that of other eukaryotes are composed of four rRNA strands while bacterial ribosomes are made of three strands of rRNA. These rRNA are coupled with proteins that form the overall structure of a ribosome. In eukaryotes, the ribosomal protein enters the nucleus and is processed together with the rRNA strands to form a large ribosome subunit and a small ribosome subunit. They are shipped out of the pores of the nucleus and meet to form the completed ribosome.
(snip)
Our cells and other animal cells contain many different components that function together to promote the wellbeing of the cell and the body. The mitochondria, for instance, provides our cells with the necessary amount of energy compounds needed to perform their various functions. The nucleus houses the DNA that cells use when they need to use genes to create proteins and regulate various systems in the body, like growth.
etc
Biology isn't my thing, but from what little I've read, I'd almost be confident to say that getting frog ribosomes would be useless for helping a sick human. So, which is more plausible - any number of compatible Vulcans? Or no compatible Vulcans but the sheer luck in becoming allies with a completely different species, one with more complex DNA and an exoskeltal cranial plate and numerous redundant organs for the other plot contrivance-heavy episode, now makes such a near-impossible feat easy-as-pie just so long as Worf agrees?
Or, if nothing else, I need some fresh ribosomes so will I go down to the blood bank or do I go down the rabbit hole and see if Bugs has any to spare - and lo and behold, his is far more compatible cuz of all them carrots he ate because Worf's prune juice made him compatible for the Romulan too... Well, maybe, since xenotransfusion is a genuine concept dating back to the 17th century and all... how's that for a plot twist? A plot twist that could have been said in the episode to make the contrivance for Worf's big butch moralized dilemma a lot more plausible in the story's own narrative? (and even then, the chances of no Vulcan being compatible seems far smaller... it's all good. Especially for the pig donors with type-O blood, where compatibility is far more likely to be viable...

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